runtime: add execution tracing functionality

This is first patch of series of patches that implement tracing functionality.
Design doc:
https://docs.google.com/document/u/1/d/1FP5apqzBgr7ahCCgFO-yoVhk4YZrNIDNf9RybngBc14/pub
Full change:
https://codereview.appspot.com/146920043

Change-Id: I84588348bb05a6f6a102c230f3bca6380a3419fe
Reviewed-on: https://go-review.googlesource.com/1450Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/runtime2.go

@@ -213,6 +213,7 @@ type g struct {
 	sigcode1     uintptr
 	sigpc        uintptr
 	gopc         uintptr // pc of go statement that created this goroutine
+	startpc      uintptr // pc of goroutine function
 	racectx      uintptr
 	waiting      *sudog // sudog structures this g is waiting on (that have a valid elem ptr)
 }
@@ -324,6 +325,8 @@ type p struct {
 	gfree    *g
 	gfreecnt int32
 
+	tracebuf *traceBuf
+
 	pad [64]byte
 }