[cmake] Teach the standalone CMake build to find the Python interpreter

the same way the LLVM CMake build does, notably using the proper CMake
module and specifically requesting an older Python version. LLDB relies
pretty heavily on not using Python 3 at this point, and without this
patch it ends up trying to use Python 3 which ends quite badly. =] With
this, I'm able to build LLDB in its standalone mode successfully on
Linux when I have both Python 2.7 and Python 3.3 installed.

llvm-svn: 224929