undo CL 14231047 / 2f4c2dde2756

undone because the change slows down profile collection
significantly and unpredictable at times (see comments
at https://golang.org/cl/14231047 for details)

««« original CL description
runtime: collect profiles even while on g0 stack

Fixes #6417

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14231047
»»»

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14535046

src/pkg/runtime/os_windows.c

@@ -402,7 +402,7 @@ profilem(M *mp)
 		tls = runtime·tls0;
 	gp = *(G**)tls;
 
-	if(gp != nil) {
+	if(gp != nil && gp != mp->g0 && gp->status != Gsyscall) {
 		// align Context to 16 bytes
 		r = (Context*)((uintptr)(&rbuf[15]) & ~15);
 		r->ContextFlags = CONTEXT_CONTROL;