lit: Set a base directory for compiler-rt tests

Setting this parameter enables llvm-lit to run on source directories for
compiler-rt test suites that implement magic in their lit.cfg.

<rdar://problem/16458307>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205262 91177308-0d34-0410-b5e6-96231b3b80d8