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.

Change-Id: I8a00f6e7b303f338ddde269b16aed0b3ede7a478
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/427935
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

libgo/runtime/go-callers.c

diff --git a/libgo/runtime/go-callers.c b/libgo/runtime/go-callers.c
index 3178369..1d4dee6 100644
--- a/libgo/runtime/go-callers.c
+++ b/libgo/runtime/go-callers.c
@@ -107,7 +107,7 @@
       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)