cmd/go: include AR env var in gccgo build IDs

The gccgo toolchain uses the archiver specified by the AR environment
variable, or `ar` by default. Teach the build ID to take the value of
this environment variable into account, since different archivers can
produce different results.

Fix #30046.

Change-Id: Ia6821258d54eecedb9026afc38a515cd564c45cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/160897


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>