cmd/compile/internal/amd64: more robust detection of GOAMD64=v1

cmd/compile/internal/amd64: more robust detection of GOAMD64=v1

Using build tool tags instead of relying on GOAMD64 env, which can be
overriden at make.bash time.

Change-Id: I96bb4b7416ea0bd1ab552feec9ede45b89a584c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/422614
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>