go/build: reserve GOARCH values for all common architectures

Whenever we introduce a new GOARCH, older Go releases won't
recognize them and this causes trouble for both our users and
us (we need to add unnecessary build tags).

Go 1.5 has introduced three new GOARCHes so far: arm64 ppc64
ppc64le, we can take the time to introduce GOARCHes for all
common architectures that Go might support in the future to
avoid the problem.

Fixes #10165.

Change-Id: Ida4f9112897cfb1e85b06538db79125955ad0f4c
Reviewed-on: https://go-review.googlesource.com/9644


Reviewed-by: Ian Lance Taylor <iant@golang.org>