debug/gdb: fix $need_expat_src usage for native-gdb

For the native-gdb (ie on the target), we unconditionally
need to build expat.

Make it a backend, it makes a litle bit cleaner code.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.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