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>

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end