[compiler-rt] Add support for arm64 macOS

Summary: Allow compiler-rt to build for arm64 support on macOS.

Reviewers: arphaman, doug.gregor, dexonsmith

Reviewed By: arphaman, dexonsmith

Subscribers: dberris, mgorny, kristof.beyls, jkorous, ributzka, danielkiss, #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D82610