runtime: show m stack during crash on m stack

The various throwing > 0 finish a change started
in a previous CL, which sets throwing = -1 to mean
"don't show the internals". That gets set during the
"all goroutines are asleep - deadlock!" crash, and it
should also be set during any other expected crash
that does not indicate a problem within the runtime.

Most runtime.throw do indicate a problem within the
runtime, however, so we should be able to enumerate
the ones that should be silent. The goroutine sleeping
deadlock is the only one I can think of.

Update #5139

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13662043

src/pkg/runtime/panic.c

@@ -443,7 +443,7 @@ runtime·dopanic(int32 unused)
 			runtime·printf("\n");
 			runtime·goroutineheader(g);
 			runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
-		} else if(t >= 2) {
+		} else if(t >= 2 || m->throwing > 0) {
 			runtime·printf("\nruntime stack:\n");
 			runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
 		}

src/pkg/runtime/traceback_arm.c

@@ -153,7 +153,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 				runtime·printf("\t%S:%d", file, line);
 				if(frame.pc > f->entry)
 					runtime·printf(" +%p", (uintptr)(frame.pc - f->entry));
-				if(m->throwing && gp == m->curg)
+				if(m->throwing > 0 && gp == m->curg)
 					runtime·printf(" fp=%p", frame.fp);
 				runtime·printf("\n");
 				nprint++;

src/pkg/runtime/traceback_x86.c

@@ -170,7 +170,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 				runtime·printf("\t%S:%d", file, line);
 				if(frame.pc > f->entry)
 					runtime·printf(" +%p", (uintptr)(frame.pc - f->entry));
-				if(m->throwing && gp == m->curg)
+				if(m->throwing > 0 && gp == m->curg)
 					runtime·printf(" fp=%p", frame.fp);
 				runtime·printf("\n");
 				nprint++;