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/PowerPC/GDBStub.h

@@ -30,6 +30,7 @@ typedef enum {
 } gdb_bp_type;
 
 void gdb_init(u32 port);
+void gdb_init_local(const char *socket);
 void gdb_deinit();
 bool gdb_active();
 void gdb_break();