cmd/go: add GOAMD64 environment variable

The variable represents the microarchitecture level for which to compile.
Valid values are v1 (default), v2, v3, v4.

Updates #45453

Change-Id: I095197fc9239d79f98896d7e745e2341354daca4
GitHub-Last-Rev: f83ed17204606264073be5b9831f9d24f2f9dbc4
GitHub-Pull-Request: golang/go#48359
Reviewed-on: https://go-review.googlesource.com/c/go/+/349595


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>