Golang dep tool workaround on AArch64

Currently there's no official `dep` tool release for arm64, so we
use `go get -u github.com/golang/dep/cmd/dep` command instead of
`curl` official release to workaround this issue, order to support
native build on arm64 server. Will remove this once official arm64
release of `dep` announced.

Signed-off-by: Dennis Chen <dennis.chen@arm.com>

build/makelib/golang.mk

@@ -172,9 +172,15 @@ go.vendor: $(DEP)
 
 $(DEP):
 	@echo === installing dep
-	@mkdir -p $(TOOLS_HOST_DIR)
-	@curl -sL -o $(DEP) https://github.com/golang/dep/releases/download/$(DEP_VERSION)/dep-$(GOHOSTOS)-$(GOHOSTARCH)
+	@mkdir -p $(TOOLS_HOST_DIR)/tmp
+	@if [ "$(GOHOSTARCH)" = "arm64" ]; then\
+		GOPATH=$(TOOLS_HOST_DIR)/tmp GOBIN=$(TOOLS_HOST_DIR) $(GOHOST) get -u github.com/golang/dep/cmd/dep;\
+		mv $(TOOLS_HOST_DIR)/dep $@;\
+	else \
+		curl -sL -o $(DEP) https://github.com/golang/dep/releases/download/$(DEP_VERSION)/dep-$(GOHOSTOS)-$(GOHOSTARCH);\
+	fi
 	@chmod +x $(DEP)
+	@rm -fr $(TOOLS_HOST_DIR)/tmp
 
 $(GOLINT):
 	@echo === installing golint