[compiler-rt][Darwin] Add arm64 to simulator platforms

I was looking around and noticed that builtins for iossim, tvossim
and watchossim was missing arm64 builds, while apple's clang
toolchain ship with these. After a bit of searching around it just
seems like these are not listed correctly in CMake to be enabled.

I enabled just arm64 since I saw that Apple clang didn't include
arm64e.

Reviewed By: t.p.northover

Differential Revision: https://reviews.llvm.org/D118759