debug/gdb: install gdbinit sample file

gdb needs to know where to find the libstdc++ helper python script
to do, well, whatever it has to do with it...

We can't install that in the user's ~/.gdbinit, it's too complex to
handle all the cases. Moreover, if the user is using more than one
toolchain, we can't put all that stuff in there...

Just provide a sample config file the user can adapt to his/her
own needs.

Thanks go to Khem RAJ for providing such a hint:
http://sourceware.org/ml/crossgcc/2011-07/msg00026.html

Reported-by: ANDY KENNEDY <ANDY.KENNEDY@adtran.com>
CC: Khem Raj <raj.khem@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

   ./configure --help

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://ymorin.is-a-geek.org/projects/crosstool

 Aloha!