This sets up a relative path symlink to the XCode sysroot.

Bug: chromium:1100006
Change-Id: I04bd8ff5158f6e00a91391e6a49530cf1a46f6eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2269452Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68573}