gdb-bridge: improve stop/step/continue integration
gdb-bridge: improve stop/step/continue integration

- track when gdb thinks we're running (and thus it's stopped) and
  aggressively poll the target state (~10ms) to notice when it
  stops and send a stop message when that happens
- treat 's' as a silent command, like 'c'
- provide signal_gdb_server() for eventual integration with
  the debugger core monitoring service

swetland/m3dev,swetland/m3dev,swetland/mdebug