lit: multiprocessing platform fix attempt

The error raised by Python varies by platform(!), so let's just catch any
exception and fall back.

Thanks to Sylvestre Ledru for noticing this on a Debian / Python 2.7 system
running code coverage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193516 91177308-0d34-0410-b5e6-96231b3b80d8