Some GCM-related changes

* cellGcmGetTiledPitchSize (thanks 3141card), cellGcmFunc15.
* sysPrxForUser: _sys_memset
* Created headers cellGcmSys.h and sysPrxForUser.h
* `int`, `int32_t` -> `s32` and `uint32_t` -> `u32`.
* Renamed Emu/GS -> Emu/RSX.