makefile: Set CC explicitly in go build (#9784)

This is required because Go does not pull CC from the make variable. This uses
whatever Go's default CC unless CC is overridden, as it is for the ARM targets.

This also makes it easier to build Nomad on a native ARM device, via:

```
make CC= pkg/linux_arm/nomad
```

GNUmakefile

@@ -55,6 +55,7 @@ SUPPORTED_OSES = Darwin Linux FreeBSD Windows MSYS_NT
 -include GNUMakefile.local
 
 pkg/%/nomad: GO_OUT ?= $@
+pkg/%/nomad: CC ?= $(shell go env CC)
 pkg/%/nomad: ## Build Nomad for GOOS_GOARCH, e.g. pkg/linux_amd64/nomad
 ifeq (,$(findstring $(THIS_OS),$(SUPPORTED_OSES)))
 	$(warning WARNING: Building Nomad is only supported on $(SUPPORTED_OSES); not $(THIS_OS))
@@ -63,6 +64,7 @@ endif
 	@CGO_ENABLED=1 \
 		GOOS=$(firstword $(subst _, ,$*)) \
 		GOARCH=$(lastword $(subst _, ,$*)) \
+		CC=$(CC) \
 		go build -trimpath -ldflags $(GO_LDFLAGS) -tags "$(GO_TAGS)" -o $(GO_OUT)
 
 pkg/linux_arm/nomad: CC = arm-linux-gnueabihf-gcc-5