[lit] Use multiprocessing based parallelism by default, on Unix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193279 91177308-0d34-0410-b5e6-96231b3b80d8
[lit] Stop hacking the GIL check interval.

 - This was never a big win, and is irrelevant now that we commonly use
   multiprocessing based parallelism.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193280 91177308-0d34-0410-b5e6-96231b3b80d8