Add platform target to Makefile (knative#536)

Makefile

@@ -14,8 +14,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-CGO_ENABLED=0
-GOOS=linux
+CGO_ENABLED=1
+GOOS ?=
+GOARCH ?=
 TEST_IMAGES=./test/test_images/helloworld knative.dev/serving/test/test_images/grpc-ping knative.dev/serving/test/test_images/multicontainer/servingcontainer knative.dev/serving/test/test_images/multicontainer/sidecarcontainer
 TEST=
 TEST_IMAGE_TAG ?= latest
@@ -28,6 +29,10 @@ build:
 	GOFLAGS='' ./hack/build.sh -f
 .PHONY: build
 
+build-with-platform:
+	./hack/build.sh -p $(GOOS) $(GOARCH)
+.PHONY: build-with-platform
+
 build-cross:
 	GOFLAGS='' ./hack/build.sh -x
 .PHONY: build-cross