arm64: Define stage_entry as weak symbol

arm64: Define stage_entry as weak symbol

This allows SoCs/CPUs to have custom stage_entry in order to apply any
fixups that need to run before standard cpu reset procedure.

BUG=chrome-os-partner:41877
BRANCH=None
TEST=Compiles successfully

Change-Id: Iaae7636349140664b19e81b0082017b63b13f45b
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 498d04b0e9a3394943f03cad603c30ae8b3805d4
Original-Change-Id: I9a005502d4cfcb76017dcae3a655efc0c8814a93
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/284867
Original-Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/10897
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>