[AArch64] Add support for __builtin_ms_va_list on aarch64

Move builtins from the x86 specific scope into the global
scope. Their use is still limited to x86_64 and aarch64 though.

This allows wine on aarch64 to properly handle variadic functions.

Differential Revision: https://reviews.llvm.org/D34475

llvm-svn: 308218