[lit] Use multiprocessing by default on Windows

Apparently nobody evaluated multiprocessing on Windows since Daniel
enabled multiprocessing on Unix in r193279. It works so far as I can
tell.

Today this is worth about an 8x speedup (631.29s to 73.25s) on my 24
core Windows machine. Hopefully this will improve Windows buildbot cycle
time, where currently it takes more time to run check-all than it does
to self-host with assertions enabled:
http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015/builds/20
  build stage 2 ninja all ( 28 mins, 22 secs )
  ninja check 2 stage 2   ( 37 mins, 38 secs )

llvm-svn: 280382