[PATCH] lib/decompress_unxz.c: removing all memory helper functions

We could do that.  But according to the comment in the original
implementation, there seems to be a concern regarding its performance
speed. If you believe all archs' memcpy would give comparable
performance to the memmove. then I could do that.

Also how about the case for overlapping buffer, especially when the
destination is at a higher address?  I do not believe memcpy guarantee
to work.

Thanks,
Mak.