A lot of the internal library headers don't have include guards because
they aren't needed. It might look like a bug, though, and it doesn't
hurt to add them. So do this.
Update https://github.com/ebiggers/libdeflate/issues/117
Move the x86 and ARM-specific code into their own directories to prevent
it from cluttering up the main library. This will make it a bit easier
to add new architecture-specific code.
But to avoid complicating things too much for people who aren't using
the provided Makefile, we still just compile all .c files for all
architectures (irrelevant ones end up #ifdef'ed out), and the headers
are included explicitly for each architecture so that an
architecture-specific include path isn't needed. So, now people just
need to compile both lib/*.c and lib/*/*.c instead of only lib/*.c.
