go/build: add GO$GOARCH-based ToolTags

go/build: add GO$GOARCH-based ToolTags

Implement proposal #45454, providing build tags based on the
sub-architecture information in the GO$GOARCH variable
(for example, GOARM for GOARCH=arm).

For example, when GOAMD64=v2, the additional build tags
amd64.v1 and amd64.v2 are defined to be true.

Fixes #45454.

Change-Id: I7be56060d47fc61843b97fd8a78498e8202c1ee7
Reviewed-on: https://go-review.googlesource.com/c/go/+/421434
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>