runtime: Don't use filename without '/' for backtrace library.

Fixes http://golang.org/issue/6715.

From-SVN: r204828

libgo/runtime/go-caller.c

@@ -101,6 +101,13 @@ __go_get_backtrace_state ()
       const char *filename;
 
       filename = (const char *) runtime_progname ();
+
+      /* If there is no '/' in FILENAME, it was found on PATH, and
+	 might not be the same as the file with the same name in the
+	 current directory.  */
+      if (__builtin_strchr (filename, '/') == NULL)
+	filename = NULL;
+
       back_state = backtrace_create_state (filename, 1, error_callback, NULL);
     }
   runtime_unlock (&back_state_lock);