Handle the AR environment variable, used by gccgo,
the same way we handle the CC environment variable.
Change-Id: I4f42161469392f68f0b5adeb9c8b52359d5108a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/526275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Bryan Mills <bcmills@google.com>