* guix/build-system/go.scm (go-target): New procedure.
(go-build): Add goarch, goos keywords. Adjust bag depending if doing a
native or cross compile.
(go-cross-build): New procedure.
* guix/build/go-build-system.scm (setup-go-environment): Accept goarch,
goos keywords. Set go environment variables based on target architecture.
* doc/guix.texi (Build Systems): Mention new go-build-system keywords.
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