Use __aarch64__ instead of __arm64__ to detect AArch64

The latter name was used by Apple in their internal AArch64 LLVM backend,
but has since been merged into LLVM upstream and renamed to AArch64.

https://github.com/llvm-mirror/llvm/commit/29f94c72014eaa5d0d3b920686e68



Change-Id: I319f42f07c95dfbcd121134fbe6e554e2d36453d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>