[gdb/build] Handle gdbsupport and gdbserver in gdb.pot

Currently, when making gdb.pot:
...
$ cd build/gdb
$ make po/gdb.pot
test -d po || mkdir po
sh -e /data/vries/gdb/src/gdb/po/gdbtext /usr/bin/xgettext gdb . /data/vries/gdb/src/gdb
$
...
only strings from build/gdb and gdb are added.

Also add strings from gdbsupport and gdbserver.

Update gdb/po/gdbtext to handle the .cc files in gdbsupport and gdbserver.

Approved-By: Tom Tromey <tom@tromey.com>