GSP: Fixed to use real shared memory object, various cleanups.

- Previously, used a hard-coded shared memory handle of 0x10002000 (as used by libctru homebrew)

GSP: Added name for shared memory.

GSP: Cleaned up assertion message.
Memory: Removed deprecated MapBlock_Shared function.
SharedMemory: Updated MapSharedMemory to use an enum for permissions.

- Also added some safety checks to MapSharedMemory.