[Sanitizers] Modified __aarch64__ to use the 64 bit version of the allocator.

This change will switch SizeClassAllocator32 to SizeClassAllocator64 on ARM. This might potentially affect ARM platforms with 39-bit address space. This addresses [[ https://github.com/google/sanitizers/issues/703  | issues/703  ]], but unlike [[ https://reviews.llvm.org/D60243 | D60243 ]] it defaults to 64 bit allocator.

Reviewed By: vitalybuka, MaskRay

Differential Revision: https://reviews.llvm.org/D137136