This commit adapts cmd/internal/buildid and cmd/go to allow the use of
gccgo on AIX.
Buildid is supported only for AIX archives.

Change-Id: I14c790a8994ae8d2ee629d8751e04189c30ffd94
Reviewed-on: https://go-review.googlesource.com/c/145417
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>