Ben Gamari authored Jan 22, 2021 and Marge Bot committed Apr 05, 2021

To support proper parsing of arm64 targets, we needed to adjust
the GHC_LLVM_TARGET function to allow parsing arm64-apple-darwin
into aarch64. This however discared the proper os detection.

To rectify this, we'll pull the os detection into separate block.

Fixes #19173.