libretro: Add GET_CORE_DATA and SET_CORE_DATA environment callbacks

[RobLoach]

This introduces two environment callbacks that allow setting a user data pointer for arbitrary core data. This is one step closer to allowing cores to avoid declaring their own global/static variables, or playing around with threads to determine their own local state.

Example in libretro-samples: libretro/libretro-samples#30

typedef struct core_data_t {
    uint32_t *frame_buf;
} core_data_t;

// Using RETRO_ENVIRONMENT_SET_CORE_DATA
core_data_t* core_data = calloc(1, sizeof(core_data_t));
core_data->frame_buf = calloc(320 * 240, sizeof(uint32_t));
environ_cb(RETRO_ENVIRONMENT_SET_CORE_DATA, core_data);

// Using RETRO_ENVIRONMENT_GET_CORE_DATA
core_data_t* core_data = NULL;
environ_cb(RETRO_ENVIRONMENT_GET_CORE_DATA, &core_data);
// Do something with core_data->frame_buf!