Reapply "[lit] Implement builtin umask (#94621)" (#155850)
This reverts commit faa4e35c622c13c7a565b979a6676d6cf3040cd4.
This was originally reverted because it was using a Python 3.9 feature
(umask in subprocess.Popen) when LLVM only requires Python 3.8. This
patch uses os.umask instead, which has been around for longer.
