[darwin] build and link with a separate compiler-rt builtins library

for device simulators

This change separates out the iOS/tvOS/watchOS simulator slices from the "libclang_rt.<os>.a"
fat archive, by moving them out to their own "libclang_rt.<os>sim.a" static archive.
This allows us to build and to link with an arm64 device simulator slice for the simulators running
on Apple Silicons, and to distribute it in one archive alongside the Intel simulator slices.

Differential Revision: https://reviews.llvm.org/D84564