tracing/runtime: Try to add support for Go1.12

Generate g for Go 1.12; no changes here compared to Go 1.11, however
go1.12 build build fails because of +build in .s file:

	# lab.nexedi.com/kirr/go123/tracing/internal/xruntime
	./runtime_g_amd64.s:3:1: +build comment must appear before package clause and be followed by a blank line

Build problem reported upstream:

	https://github.com/golang/go/issues/31410

tracing/internal/xruntime/g_typedef

@@ -87,7 +87,7 @@ gen_zruntime() {
 
 
 # main driver
-gov="go18 go19 go1.10 go1.11"
+gov="go18 go19 go1.10 go1.11 go1.12"
 
 for g in $gov; do
 	goset $g

tracing/internal/xruntime/zruntime_g_go1.12.go

+// Code generated by g_typedef; DO NOT EDIT.
+
+// +build go1.12,!go1.13
+
+package xruntime
+
+import "unsafe"
+
+type g struct {
+	// Stack parameters.
+	// stack describes the actual stack memory: [stack.lo, stack.hi).
+	// stackguard0 is the stack pointer compared in the Go stack growth prologue.
+	// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
+	// stackguard1 is the stack pointer compared in the C stack growth prologue.
+	// It is stack.lo+StackGuard on g0 and gsignal stacks.
+	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
+	stack       stack   // offset known to runtime/cgo
+	stackguard0 uintptr // offset known to liblink
+	stackguard1 uintptr // offset known to liblink
+
+	_panic         *_panic // innermost panic - offset known to liblink
+	_defer         *_defer // innermost defer
+	m              *m      // current m; offset known to arm liblink
+	sched          gobuf
+	syscallsp      uintptr        // if status==Gsyscall, syscallsp = sched.sp to use during gc
+	syscallpc      uintptr        // if status==Gsyscall, syscallpc = sched.pc to use during gc
+	stktopsp       uintptr        // expected sp at top of stack, to check in traceback
+	param          unsafe.Pointer // passed parameter on wakeup
+	atomicstatus   uint32
+	stackLock      uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
+	goid           int64
+	schedlink      guintptr
+	waitsince      int64      // approx time when the g become blocked
+	waitreason     waitReason // if status==Gwaiting
+	preempt        bool       // preemption signal, duplicates stackguard0 = stackpreempt
+	paniconfault   bool       // panic (instead of crash) on unexpected fault address
+	preemptscan    bool       // preempted g does scan for gc
+	gcscandone     bool       // g has scanned stack; protected by _Gscan bit in status
+	gcscanvalid    bool       // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
+	throwsplit     bool       // must not split stack
+	raceignore     int8       // ignore race detection events
+	sysblocktraced bool       // StartTrace has emitted EvGoInSyscall about this goroutine
+	sysexitticks   int64      // cputicks when syscall has returned (for tracing)
+	traceseq       uint64     // trace event sequencer
+	tracelastp     puintptr   // last P emitted an event for this goroutine
+	lockedm        muintptr
+	sig            uint32
+	writebuf       []byte
+	sigcode0       uintptr
+	sigcode1       uintptr
+	sigpc          uintptr
+	gopc           uintptr         // pc of go statement that created this goroutine
+	ancestors      *[]ancestorInfo // ancestor information goroutine(s) that created this goroutine (only used if debug.tracebackancestors)
+	startpc        uintptr         // pc of goroutine function
+	racectx        uintptr
+	waiting        *sudog         // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
+	cgoCtxt        []uintptr      // cgo traceback context
+	labels         unsafe.Pointer // profiler labels
+	timer          *timer         // cached timer for time.Sleep
+	selectDone     uint32         // are we participating in a select and did someone win the race?
+
+	// gcAssistBytes is this G's GC assist credit in terms of
+	// bytes allocated. If this is positive, then the G has credit
+	// to allocate gcAssistBytes bytes without assisting. If this
+	// is negative, then the G must correct this by performing
+	// scan work. We track this in bytes to make it fast to update
+	// and check for debt in the malloc hot path. The assist ratio
+	// determines how this corresponds to scan work debt.
+	gcAssistBytes int64
+}
+type stack struct {
+	lo uintptr
+	hi uintptr
+}
+type _panic struct {
+	argp      unsafe.Pointer // pointer to arguments of deferred call run during panic; cannot move - known to liblink
+	arg       interface{}    // argument to panic
+	link      *_panic        // link to earlier panic
+	recovered bool           // whether this panic is over
+	aborted   bool           // the panic was aborted
+}
+type _defer struct {
+	siz     int32
+	started bool
+	sp      uintptr // sp at time of defer
+	pc      uintptr
+	fn      *funcval
+	_panic  *_panic // panic that is running defer
+	link    *_defer
+}
+type gobuf struct {
+	// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
+	//
+	// ctxt is unusual with respect to GC: it may be a
+	// heap-allocated funcval, so GC needs to track it, but it
+	// needs to be set and cleared from assembly, where it's
+	// difficult to have write barriers. However, ctxt is really a
+	// saved, live register, and we only ever exchange it between
+	// the real register and the gobuf. Hence, we treat it as a
+	// root during stack scanning, which means assembly that saves
+	// and restores it doesn't need write barriers. It's still
+	// typed as a pointer so that any other writes from Go get
+	// write barriers.
+	sp   uintptr
+	pc   uintptr
+	g    guintptr
+	ctxt unsafe.Pointer
+	ret  uintreg
+	lr   uintptr
+	bp   uintptr // for GOEXPERIMENT=framepointer
+}
+type funcval struct {
+	fn uintptr
+}
+type timer struct {
+	tb *timersBucket // the bucket the timer lives in
+	i  int           // heap index
+
+	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
+	// each time calling f(arg, now) in the timer goroutine, so f must be
+	// a well-behaved function and not block.
+	when   int64
+	period int64
+	f      func(interface{}, uintptr)
+	arg    interface{}
+	seq    uintptr
+}
+type guintptr uintptr
+type puintptr uintptr
+type muintptr uintptr
+type waitReason uint8
+type ancestorInfo struct {
+	pcs  []uintptr // pcs from the stack of this goroutine
+	goid int64     // goroutine id of this goroutine; original goroutine possibly dead
+	gopc uintptr   // pc of go statement that created this goroutine
+}
+type uintreg  uint	// FIXME wrong on amd64p32
+type m struct {}		// FIXME stub
+type sudog struct {}	// FIXME stub
+type timersBucket struct {}	// FIXME stub