lit: Issue a note when multiprocessing fails to load

If multiprocessing was requested, detected as available and subsequently failed
to initialize it's worth letting the user know about it before falling back to
threads.

This condition can arise in certain OpenBSD / FreeBSD Python versions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193465 91177308-0d34-0410-b5e6-96231b3b80d8