gdb: bump to 7.3.1

this version fixes compilation issue on some old build systems like
openSUSE 10.3 saying some host libraries were too old

[Peter: drop bugfix number from config name, similar to kernel-headers]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>