Use x/sys/unix epoll (#737)

* Make gobgp compile in an image

This patch adds Makefile logic, similar to the kube-router target,
allowing gobgp to be built in a container.

* Use unix.Epoll* functions

To be able to compile and run on the Linux@arm64 architecture
one has to use the `golang.org/x/sys/unix` package instead
the `syscall` package. This is because of these Go upstream
bugs that won't be fixed in the standard library:

- syscall: arm64: epoll_wait syscall not implemented
  golang/go#25813

Author: dparalen

Makefile

@@ -208,8 +208,16 @@ else
 endif
 
 gobgp:
+ifeq "$(BUILD_IN_DOCKER)" "true"
+	@echo Building gobgp
+	$(DOCKER) run -v $(PWD)/vendor:/go/src -w /go/src/github.com/osrg/gobgp/gobgp $(DOCKER_BUILD_IMAGE) \
+    sh -c 'GOARCH=$(GOARCH) CGO_ENABLED=0 go build -o gobgp'
+	@echo Finished building gobgp.
+else
 	cd vendor/github.com/osrg/gobgp/gobgp && \
-	CGO_ENABLED=0 GOARCH=$(GOARCH) GOOS=linux go build -o $(MAKEFILE_DIR)/gobgp
+	CGO_ENABLED=0 GOARCH=$(GOARCH) GOOS=linux go build -o gobgp
+endif
+	cp -f vendor/github.com/osrg/gobgp/gobgp/gobgp gobgp
 
 multiarch-binverify:
 	@echo 'Verifying kube-router gobgp for ARCH=$(FILE_ARCH) ...'