Add support for the GDB Python Library

[Joel Brobecker]

Hello,

One of the things that we still don't have in the FSF tree is a "GDB
Python Library" - that is a collection of standard Python scripts that
come with GDB. The idea is to help future contributions but setting
things up so that it's easy to add new python-based commands/functions/
features, etc.

This is what this couple of patches provide:
  - Patch #1: Defines a pythondir where the GDB Python library is
    expected to be found, and enhances the GDB/python support to
    look for them there.
  - Path #2: is built on top of patch #1, and provides the first
    such python patch.  We selected one form project archer that
    might be useful: pahole.py.