The flag "mac_use_goma_rbe" creates a symlink to the Xcode SDK
directory, under $root_build_dir/sdk/xcode_links.
e.g. the -isysroot arg in the build command becomes:
-isysroot sdk/xcode_links/MacOSX10.15.sdk
This is based on a similar solution for Chromium iOS builders:
http://crrev.com/c/1946174http://crrev.com/c/2102649
Bug: b/151697301
Change-Id: Ie65d3ae2b98da5977ad0f5a75f865c838bbbb963
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139782
Commit-Queue: Simon Que <sque@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>