Fold a copy into the adler32 function for UPDATEWINDOW for neon

Fold a copy into the adler32 function for UPDATEWINDOW for neon

So a lot of alterations had to be done to make this not worse and
so far, it's not really better, either. I had to force inlining for
the adler routine, I had to remove the x4 load instruction otherwise
pipelining stalled, and I had to use restrict pointers with a copy
idiom for GCC to inline a copy routine for the tail.

Still, we see a small benefit in benchmarks, particularly when done
with size of our window or larger. There's also an added benefit that
this will fix #1824.