Fix build issues on BSDs in pkg/cpu (#7116)

Also add a cross compile script to test always cross
compilation for some well known platforms and architectures
, we support out of box compilation of these platforms even
if we don't make an official release build.

This script is to avoid regressions in this area when we
add platform dependent code.

Makefile

@@ -19,6 +19,9 @@ getdeps:
 	@echo "Installing misspell" && go get -u github.com/client9/misspell/cmd/misspell
 	@echo "Installing ineffassign" && go get -u github.com/gordonklaus/ineffassign
 
+crosscompile:
+	@(env bash $(PWD)/buildscripts/cross-compile.sh)
+
 verifiers: getdeps vet fmt lint cyclo deadcode spelling
 
 vet: