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>
InstrProf: lit.cfg magic for running tests from source

Implement magic in compiler-rt to enable llvm-lit to be invoked on the
source tree in lib/profile.  This relies on a paired commit in the llvm
tree that exposes a new built-in parameter.

<rdar://problem/16458307>