GDB stub over UNIX socket

This is available with the `GDBSocket` option in
`~/.dolphin-emu/Config/Dolphin.ini`.

GDB can connect to it with:

    $ powerpc-eabi-gdb
    (gdb) target remote |socat STDIO UNIX:foo.sock

Because I don't like so much binding the GDB stub socket to 0.0.0.0.
On Linux, with a suitable umask, we can make sure that another local
user cannot connect to the socket.

Source/Core/Core/CoreParameter.h

@@ -133,6 +133,9 @@ struct SCoreStartupParameter
 	bool bEnableDebugging;
 	#ifdef USE_GDBSTUB
 	int  iGDBPort;
+	#ifndef _WIN32
+	std::string gdb_socket;
+	#endif
 	#endif
 	bool bAutomaticStart;
 	bool bBootToPause;