Only enable race detector when GOHOSTARCH is amd64.

This enables native builds to still run the test and all targets without
problems on say 386.

Build failure on Buildkite build 85, prevents enabling native FreeBSD
386 builds.

Makefile

@@ -14,6 +14,7 @@
 GO     ?= GO15VENDOREXPERIMENT=1 go
 GOPATH := $(firstword $(subst :, ,$(shell $(GO) env GOPATH)))
 GOARCH := $(shell $(GO) env GOARCH)
+GOHOSTARCH := $(shell $(GO) env GOHOSTARCH)
 
 PROMU       ?= $(GOPATH)/bin/promu
 STATICCHECK ?= $(GOPATH)/bin/staticcheck
@@ -26,6 +27,11 @@ DOCKER_IMAGE_TAG        ?= $(subst /,-,$(shell git rev-parse --abbrev-ref HEAD))
 MACH                    ?= $(shell uname -m)
 DOCKERFILE              ?= Dockerfile
 
+ifeq ($(GOHOSTARCH),amd64)
+	# Only supported on amd64
+	test-flags := -race
+endif
+
 ifeq ($(OS),Windows_NT)
     OS_detected := Windows
 else
@@ -63,7 +69,7 @@ style:
 
 test: collector/fixtures/sys/.unpacked
 	@echo ">> running tests"
-	@$(GO) test -short -race $(pkgs)
+	$(GO) test -short $(test-flags) $(pkgs)
 
 test-32bit: collector/fixtures/sys/.unpacked
 	@echo ">> running tests in 32-bit mode"