mac: Pull a prebuilt libc++-static.a.

Not used for anything yet.

BUG=400091
R=mark@chromium.org
TBR=torne

Review URL: https://codereview.chromium.org/1415213004 .

Cr-Original-Commit-Position: refs/heads/master@{#355729}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7ef5508bd7b471d7b4a67886e23feb89a3a13713