gdb: fix debugging of MIPS(el) applications

GDB previously failed with the following:

root@OpenWrt:/# gdb busybox
[snip]
(gdb) r
Starting program: /bin/busybox
GDB bug: target.c (gdb_signal_from_host): unrecognized real-time signal

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 34258