cmd/internal/ld: generate correct .debug_frames on RISC architectures

With this patch, gdb seems to be able to corretly backtrace Go
process on at least linux/{arm,arm64,ppc64}.

Change-Id: Ic40a2a70e71a19c4a92e4655710f38a807b67e9a
Reviewed-on: https://go-review.googlesource.com/9822
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime-gdb_test.go

@@ -85,7 +85,7 @@ func TestGdbPython(t *testing.T) {
 	// stack frames on RISC architectures.
 	canBackTrace := false
 	switch runtime.GOARCH {
-	case "amd64", "386":
+	case "amd64", "386", "ppc64", "ppc64le", "arm", "arm64":
 		canBackTrace = true
 		args = append(args,
 			"-ex", "echo BEGIN goroutine 2 bt\n",