cmd/go: pass -X64 to ar on aix/ppc64

On aix/ppc64, ar tool must always have -X64 argument if it aims to
create 64 bits archives.

Change-Id: I06eb4828f2815ef0df118aff47ade980080a1c74
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165317
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed