runtime: print stack of G during a signal

Sequence of operations:
- Go code does a systemstack call
- during the systemstack call, receive a signal
- signal requests a traceback of all goroutines

The orignal G is still marked as _Grunning, so the traceback code
refuses to print its stack.

Fix by allowing traceback of Gs whose caller is on the same M as G is.
G can't be modifying its stack if that is the case.

Fixes #10546

Change-Id: I2bcea48c0197fbf78ab6fa080027cd80181083ad
Reviewed-on: https://go-review.googlesource.com/9435Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/traceback.go

@@ -627,7 +627,11 @@ func tracebackothers(me *g) {
 		}
 		print("\n")
 		goroutineheader(gp)
-		if readgstatus(gp)&^_Gscan == _Grunning {
+		// Note: gp.m == g.m occurs when tracebackothers is
+		// called from a signal handler initiated during a
+		// systemstack call.  The original G is still in the
+		// running state, and we want to print its stack.
+		if gp.m != g.m && readgstatus(gp)&^_Gscan == _Grunning {
 			print("\tgoroutine running on other thread; stack unavailable\n")
 			printcreatedby(gp)
 		} else {