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>

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""