[Darwin] Fix OS version checks inside simulators

compiler-rt checks OS versions by querying the Darwin kernel version.
This is not necessarily correct inside the simulators if the simulator
runtime is not aligned with the host macOS.  Let's instead check the
`SIMULATOR_RUNTIME_VERSION` env var.

rdar://63031937

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D83977