Merging r372835:

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r372835 | mgorny | 2019-09-25 02:47:35 -0700 (Wed, 25 Sep 2019) | 12 lines

[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
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