Suppress python readline module under Linux to fix a seg fault.

Bug fix for pr18841:
http://llvm.org/bugs/show_bug.cgi?id=18841

This change creates a stub Python readline.so module that does almost
nothing. Its whole purpose is to prevent Python from loading the real
module, something it does during the embedded Python interpreter's
initialization sequence (and way before lldb ever requests it within
embedded_interpreter.py).

On Ubuntu 12.04 and 13.10 x86_64, and in the Python 2.7.6 tree, the
stock Python readline module links against the GNU readline library.
This appears to be the case on all Pythons except where __APPLE__ is
defined. LLDB now requires linking against the libedit library.
Something about having both libedit.so and libreadline.so linked into
the same process space is causing the Python readline.so to trigger a
NULL memory access. I have put in a separate patch to python.org.

This suppression of embedded interpreter readline support can be
removed if at least any one of the following happens:

1. The stock python distribution accepts a patch similar to what I
submitted to Python 2.7.6's Modules/readline.c file.

2. The stock python distribution implements Modules/readline.c in
terms of libedit's readline compatibility mode (i.e. essentially
compiles it the way __APPLE__ compiles that module) under Linux.

3. a clean-room implementation of the python readline module is
implemented against libedit (either readline compatibility mode or
native libedit). This could be implemented within the readline.cpp
file that this change introduces. It cannot be a fork of python's
readline.c module due to llvm licensing.

The net effect of this change on Linux is that the embedded python's
readline support will not exist.