Detect AArch64 architecture

Adds WTF platform support for the AArch64 architecture.

Patch is based on WebKit-gtk patch done by Riku Voipio, and was
cherry-picked and tested by Marcin Juszkiewicz.

Task-number: QTBUG-35442

Change-Id: Ie6194f3c430cb6513367a3cdf221a41d60a1ed14
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>