This enables the recipe to set FORCE_MAC_TOOLCHAIN and decide if hermetic
or system xcode is used.

TBR=tmrts@chromium.org

Bug: chromium:986701
Change-Id: I2c1ba75d5d62e493bd9dc7a5ec5f7e40465d8f98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1722557Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62947}