runtime: print runtime frames in throwsplit trace

newstack manually prints the stack trace if we try to grow the stack
when throwsplit is set. However, the default behavior is to omit
runtime frames. Since runtime frames can be critical to understanding
this crash, this change fixes this traceback to include them.

Updates #21431.

Change-Id: I5aa43f43aa2f10a8de7d67bcec743427be3a3b5d
Reviewed-on: https://go-review.googlesource.com/79518
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/stack.go

@@ -942,6 +942,7 @@ func newstack() {
 			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
 			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")
 
+		thisg.m.traceback = 2 // Include runtime frames
 		traceback(morebuf.pc, morebuf.sp, morebuf.lr, gp)
 		throw("runtime: stack split at bad time")
 	}