runtime: enable external linker tests for aix/ppc64

Change-Id: Icc42843adb15c2aba1cfea854fad049c6704344b
Reviewed-on: https://go-review.googlesource.com/c/go/+/164014
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/crash_cgo_test.go

@@ -90,9 +90,9 @@ func TestCgoExternalThreadSIGPROF(t *testing.T) {
 	case "plan9", "windows":
 		t.Skipf("no pthreads on %s", runtime.GOOS)
 	}
-	if runtime.GOARCH == "ppc64" {
+	if runtime.GOARCH == "ppc64" && runtime.GOOS == "linux" {
 		// TODO(austin) External linking not implemented on
-		// ppc64 (issue #8912)
+		// linux/ppc64 (issue #8912)
 		t.Skipf("no external linking on ppc64")
 	}