debug/gdb: use libexpat

gdb can use libexpat to parse target memory descriptions,
among other things. See:
http://sourceware.org/ml/crossgcc/2010-08/msg00168.html

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =