Improve support for compiler-rt tests in CMake build.

Now compiler-rt tests run correctly if compiler-rt is checked out into
arbitrary directory (not necessarily projects/compiler-rt).
Patch by Greg Fitzgerald!


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182726 91177308-0d34-0410-b5e6-96231b3b80d8