Developers using Avocado's GDB support may also need to use their custom
build of gdbserver.

This implements the same logic and options as the previous commit does
for the GDB binary, including API level changes.

Changes from v1:
   * use `avocado.utils.process.find_command` as the first default
     location for the gdbserver binary, falling back to the hard coded
     default of `/usr/bin/gdbserver` if that fails.
Signed-off-by: NCleber Rosa <crosa@redhat.com>