lit: handle late multiprocessing errors gracefully

This should be a better fix for lit multiprocessing failures, replacing the
OpenBSD and FreeBSD workarounds in r193413 and r193457.

Reference: http://bugs.python.org/issue3770

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193463 91177308-0d34-0410-b5e6-96231b3b80d8