gcc-source: Ensure deploy_source_date_epoch sstate hash doesn't change

Currently if you switch machines, gcc-source do_deploy_source_date_epoch
would re-run as the stamps are tune specific. This hasn't caused much
of an issue until now, however if we fix the gcc recipes to reuse the
timestamp from this task, it does then create problems.

Copy code from allarch to ensure this task hash doesn't change between
machines/tunes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 7e052d03)
Signed-off-by: Steve Sakoman <steve@sakoman.com>