cm: Use fork of aarch64 gcc for darwin as well

* 5515fd9b introduced a fork of the gcc with the A53 errata workarounds,
    but the darwin-x86 aarch64 gcc needs them as well.

Change-Id: Ia7302be4885423d52268066c7ffded9d15ff812a
Signed-off-by: Brandon McAnsh <brandon.mcansh@gmail.com>
(cherry picked from commit aa471692)
Ticket: CYNGNOS-442