runtime: ignore __morestack function in runtime.Callers

We were ignoring all functions starting with "__morestack_", but not
the function "__morestack" itself.  Without this change, some tests
such as recover.go started failing recently, though I'm not sure
exactly what changed.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/427935

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-d53e8a0e94e34dc609e34dd5e404debda2640cfb
+6543b7fc6da533eb976b37649a925e7fd5a521fa
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-callers.c

@@ -107,7 +107,7 @@ callback (void *data, uintptr_t pc, const char *filename, int lineno,
       p = function;
       if (__builtin_strncmp (p, "___", 3) == 0)
 	++p;
-      if (__builtin_strncmp (p, "__morestack_", 12) == 0)
+      if (__builtin_strncmp (p, "__morestack", 11) == 0)
 	return 0;
     }
   else if (filename != NULL)