cmd/asm/internal/arch: add package definition
cmd/asm/internal/arch: add package definition

The package arch didn't have a definition as you can see in https://tip.golang.org/pkg/cmd/asm/internal/arch/

Change-Id: I07653b396393a75c445d04dbae5e22e90a0d5133
GitHub-Last-Rev: a859e9410f38073853687b933f53eb6570af3216
GitHub-Pull-Request: golang/go#26817
Reviewed-on: https://go-review.googlesource.com/127929
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,josharian/go,josharian/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,josharian/go,josharian/go,akutz/go,CAFxX/go,nawawi/go,josharian/go,nawawi/go,josharian/go,CAFxX/go,CAFxX/go,akutz/go,golang/go,golang/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,josharian/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,akutz/go,cloudflare/go,golang/go,josharian/go,nawawi/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go