Add support for apple silicon

For istio#29596

Next step is include it in the release

Manually built at
http://storage.googleapis.com/howardjohn/tmp/istioctl-osx-arm64. Please
test it, I don't have a mac.

Makefile.core.mk

@@ -397,7 +397,9 @@ ${ISTIO_OUT}/release/istioctl-linux-armv7: depend
 ${ISTIO_OUT}/release/istioctl-linux-arm64: depend
 	GOOS=linux GOARCH=arm64 LDFLAGS=$(RELEASE_LDFLAGS) common/scripts/gobuild.sh $@ ./istioctl/cmd/istioctl
 ${ISTIO_OUT}/release/istioctl-osx: depend
-	GOOS=darwin LDFLAGS=$(RELEASE_LDFLAGS) common/scripts/gobuild.sh $@ ./istioctl/cmd/istioctl
+	GOOS=darwin GOARCH=amd64 LDFLAGS=$(RELEASE_LDFLAGS) common/scripts/gobuild.sh $@ ./istioctl/cmd/istioctl
+${ISTIO_OUT}/release/istioctl-osx-arm64: depend
+	GOOS=darwin GOARCH=arm64 LDFLAGS=$(RELEASE_LDFLAGS) common/scripts/gobuild.sh $@ ./istioctl/cmd/istioctl
 ${ISTIO_OUT}/release/istioctl-win.exe: depend
 	GOOS=windows LDFLAGS=$(RELEASE_LDFLAGS) common/scripts/gobuild.sh $@ ./istioctl/cmd/istioctl
 
@@ -418,6 +420,7 @@ binaries-test:
 .PHONY: istioctl-all
 istioctl-all: ${ISTIO_OUT}/release/istioctl-linux-amd64 ${ISTIO_OUT}/release/istioctl-linux-armv7 ${ISTIO_OUT}/release/istioctl-linux-arm64 \
 	${ISTIO_OUT}/release/istioctl-osx \
+	${ISTIO_OUT}/release/istioctl-osx-arm64 \
 	${ISTIO_OUT}/release/istioctl-win.exe
 
 .PHONY: istioctl.completion