[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.

Note that we still use the old code path as a fallback in case the
`SIMULATOR_RUNTIME_VERSION` environment variable isn't set.

rdar://63031937

Reviewers: delcypher

Differential Revision: https://reviews.llvm.org/D79979