runtime: Tweak runtime.Callers for Go 1 compatibility.

From-SVN: r187781

libgo/runtime/go-callers.c

@@ -72,5 +72,8 @@ int Callers (int, struct __go_open_array)
 int
 Callers (int skip, struct __go_open_array pc)
 {
-  return runtime_callers (skip, (uintptr *) pc.__values, pc.__count);
+  /* In the Go 1 release runtime.Callers has an off-by-one error,
+     which we can not correct because it would break backward
+     compatibility.  Adjust SKIP here to be compatible.  */
+  return runtime_callers (skip - 1, (uintptr *) pc.__values, pc.__count);
 }