[lldb] [cmake] Fix installing Python modules on systems using /usr/lib

Fix installing Python modules on systems that use /usr/lib for Python
while installing other libraries in /usr/lib64.  Rewrite CMake logic
to query correct directories from Python, similarly to how
prepare_binding_Python.py does it.  Furthermore, change the regex used
in get_relative_lib_dir.py to allow 'lib' without suffix.

I think that the code can be further improved but I'd like to take
this enterprise in smaller steps in case one of them breaks something.

Differential Revision: https://reviews.llvm.org/D67890

llvm-svn: 372835