Add gvr_shim prebuilts for use_custom_libcxx=true builds

BUG=767901
R=pcc

Change-Id: I8c85d502fb56bc163b4c7e80de00975d4fadde8e
Reviewed-on: https://chromium-review.googlesource.com/1067734
Reviewed-by: Biao She <bshe@chromium.org>
Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561257}
5 files changed