GDB: support for setting gdb/gdbserver paths [v2]

Developers using Avocado's GDB support may need to use their custom build of gdb and gdbserver, or simply their system has those at a different location.

Changes from v1:

   * use `avocado.utils.process.find_command()` as the first default location for the gdb/gdbserver binary, falling back to the hard coded defaults `/usr/bin/gdb` and `/usr/bin/gdbserver` if those fail. Suggested by @lmr.