misc/cgo/testcarchive: skip TestSignalForwardingExternal on darwin/amd64

On darwin/amd64 the runtime method sigctxt.fixsigcode changes SIGSEGV
signals so that they are never marked SI_USER. CL 169120 changed the
signal handler to call fixsigcode even when the signal is delivered to
a non-Go thread. This breaks TestSignalForwardingExternal, so skip it.

Change-Id: I6740fb5a8f4f854ca69793537a983a696da3b495
Reviewed-on: https://go-review.googlesource.com/c/go/+/170446
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testcarchive/carchive_test.go

@@ -325,6 +325,8 @@ func TestSignalForwarding(t *testing.T) {
 func TestSignalForwardingExternal(t *testing.T) {
 	if GOOS == "freebsd" || GOOS == "aix" {
 		t.Skipf("skipping on %s/%s; signal always goes to the Go runtime", GOOS, GOARCH)
+	} else if GOOS == "darwin" && GOARCH == "amd64" {
+		t.Skipf("skipping on %s/%s: runtime does not permit SI_USER SIGSEGV", GOOS, GOARCH)
 	}
 	checkSignalForwardingTest(t)