[Triple][MachO] Define "arm64e", an AArch64 subarch for Pointer Auth.

This also teaches MachO writers/readers about the MachO cpu subtype,
beyond the minimal subtype reader support present at the moment.

This also defines a preprocessor macro to allow users to distinguish
__arm64__ from __arm64e__.

arm64e defaults to an "apple-a12" CPU, which supports v8.3a, allowing
pointer-authentication codegen.
It also currently defaults to ios14 and macos11.

Differential Revision: https://reviews.llvm.org/D87095