Implement `sys::getHostCPUName()` for Darwin ARM

Summary: Currently there is no implementation of `sys::getHostCPUName()` for Darwin ARM targets. This patch makes it so that LLVM running on ARM makes reasonable guesses about the CPU features of the host CPU.

Reviewers: t.p.northover, lhames, efriedma

Reviewed By: efriedma

Subscribers: rjmccall, efriedma, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69597