runtime: don't check for stale runtime

The gccgo runtime is never stale, and on a system with gc sources in
    ~/go the test may wind up checking whether the gc runtime is stale.
    
    Reviewed-on: https://go-review.googlesource.com/102282

From-SVN: r258865

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-e9c0e4d8fd3d951a367bb6a50e5cb546e01b81a8
+3aa5fc91094c5f24b26747ec176ad44cde784fc7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/crash_test.go

@@ -150,6 +150,9 @@ var (
 
 func checkStaleRuntime(t *testing.T) {
 	staleRuntimeOnce.Do(func() {
+		if runtime.Compiler == "gccgo" {
+			return
+		}
 		// 'go run' uses the installed copy of runtime.a, which may be out of date.
 		out, err := testenv.CleanCmdEnv(exec.Command(testenv.GoToolPath(t), "list", "-gcflags=all="+os.Getenv("GO_GCFLAGS"), "-f", "{{.Stale}}", "runtime")).CombinedOutput()
 		if err != nil {