toolchain-external: support more gdbserver locations

The x86/x86-64 CodeSourcery toolchains use some weird locations for
the gdbserver binary:

$ find . -name 'gdbserver'
./i686-pc-linux-gnu/libc/atom/usr/bin/gdbserver
./i686-pc-linux-gnu/libc/atom/usr/lib/bin/gdbserver
./i686-pc-linux-gnu/libc/core2/usr/bin/gdbserver
./i686-pc-linux-gnu/libc/core2/usr/lib64/bin/gdbserver
./i686-pc-linux-gnu/libc/usr/lib/bin/gdbserver
./i686-pc-linux-gnu/libc/usr/lib64/bin/gdbserver

Notice that it's sometimes hidden in a usr/{lib,lib64}/bin
directory. This patch changes the gdbserver logic to also try in this
location.

Originally based on work done by Daniel Nilsson, visible at
http://patchwork.ozlabs.org/patch/155767/

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>