[AArch64] Predefine __AARCH64_CMODEL_*__ as GCC does

Make Clang on aarch64 targets predefine `__AARCH64_CMODEL_SMALL__`
or `__AARCH64_CMODEL_TINY__`, etc.  These are the names that GCC
uses for its predefines.

Reviewed By: tamur, MaskRay

Differential Revision: https://reviews.llvm.org/D75002