tweak ar/libtool setup

This is what I was getting at on the comments in PS 5.

Change-Id: I5435860ebea9ed01217d7acb378e1fadce19b107
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/250776
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed