Add the lit site config for unittests to bin/llvm-lit

This missing parameter was causing bin/llvm-lit to run the unittests
from my primary build directory instead of my self-hosting build
directory because llvm-config was on my PATH.

This more closely matches what 'make check' will pass to lit.py.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197444 91177308-0d34-0410-b5e6-96231b3b80d8