Improve sse41 adler32 performance

Improve sse41 adler32 performance

Rather than doing opportunistic aligned loads, we can do scalar
unaligned loads into our two halves of the checksum until we hit
alignment.  Then, we can subtract from the max number of sums for the
first run through the loop.

This allows us to force aligned loads for unaligned buffers (likely a
common case for arbitrary runs of memory). This is not meaningful after
Nehalem but pre-Nehalem architectures it makes a substantial difference
to performance and is more foolproof than hoping for an aligned buffer.

Improvement is around 44-50% for unaligned worst case scenarios.