[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