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: I81b8c43fb3a6a65c56446f84a7b1001075998d1e
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>