libgo: correctly handle AIX FAT library creation

The previous patch wasn't working everytime. Especially when AR had
"-X32_64", the new .so would replace the default one and not just being
added.

Change-Id: Ibf9a40b3b3bc41ffd88d75bdc89bfc780d34ed16
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/247377
Reviewed-by: Ian Lance Taylor <iant@golang.org>
5 files changed