set Go env vars in environment

This way, a template can overwrite do_build but still have good defaults
for GOPATH, GOARCH and GOARM.

common/environment/build-style/go.sh

@@ -3,3 +3,12 @@ if [ "$CROSS_BUILD" ]; then
 	hostmakedepends+=" go-cross-linux"
 fi
 nostrip=yes
+
+case "$XBPS_TARGET_MACHINE" in
+	armv6*) export GOARCH=arm; export GOARM=6;;
+	armv7*) export GOARCH=arm; export GOARM=7;;
+	i686*) export GOARCH=386;;
+	x86_64*) export GOARCH=amd64;;
+esac
+
+export GOPATH="${wrksrc}/_build-${pkgname}-xbps"