Fix StopTheWorld crash due to change in runtime.stopTheWorld signature:

In Go1.21 stopTheWorld changed from accepting reason as string into
accepting reason as uint8 code:

    https://github.com/golang/go/commit/b1aadd034c1f
    https://golang.org/cl/494495

which leads to the following crash if reason is still passed as string:

    fatal error: index out of range

    goroutine 6 [running]:
    runtime.throw({0x531aad?, 0x0?})
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:1077 +0x5c fp=0xc000042608 sp=0xc0000425d8 pc=0x4365bc
    runtime.panicCheck1(0x4785af?, {0x531aad, 0x12})
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:58 +0x94 fp=0xc000042628 sp=0xc000042608 pc=0x434034
    runtime.goPanicIndex(0x8d, 0x11)
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:113 +0x2e fp=0xc000042668 sp=0xc000042628 pc=0x4340ee
    runtime.stwReason.String(...)
    	/home/kirr/src/tools/go/go/src/runtime/proc.go:1217
    runtime.stopTheWorld(0x8d)
    	/home/kirr/src/tools/go/go/src/runtime/proc.go:1260 +0xe6 fp=0xc0000426c8 sp=0xc000042668 pc=0x43b3a6
    lab.nexedi.com/kirr/go123/tracing/internal/xruntime.StopTheWorld(...)
    	/home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime/runtime.go:40
    lab.nexedi.com/kirr/go123/tracing/internal/xruntime.TestStartStopTheWorld(0xc000007860)
    	/home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime/runtime_test.go:75 +0x139 fp=0xc000042770 sp=0xc0000426c8 pc=0x4fa0f9
    testing.tRunner(0xc000007860, 0x53a8b8)
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1595 +0xff fp=0xc0000427c0 sp=0xc000042770 pc=0x4bbd9f
    testing.(*T).Run.func1()
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1648 +0x25 fp=0xc0000427e0 sp=0xc0000427c0 pc=0x4bcd25
    runtime.goexit()
    	/home/kirr/src/tools/go/go/src/runtime/asm_amd64.s:1650 +0x1 fp=0xc0000427e8 sp=0xc0000427e0 pc=0x468901
    created by testing.(*T).Run in goroutine 1
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1648 +0x3ad

-> Fix it by accounting that STW reason is uint8 on go1.21 and passing it
to runtime correspondingly.

Sadly we cannot express arbitrary string reason into fixed list of
reasons that runtime and stdlib tracing are aware of, so we always use
"unknown" for custom stop-the-world requests.