[gdbserver] Add codespell section in setup.cfg

Add a codespell section in new config file gdbserver/setup.cfg, similar to the
one in gdbsupport/setup.cfg.

There's just one item left:
...
$ codespell --config ./gdbserver/setup.cfg gdbserver
gdbserver/xtensa-xtregs.cc:34: siz ==> size, six
...