lib/raid6: sort algos in rough performance order

Sort the list of RAID6 algorithms in roughly decreasing order of
expected performance: newer instruction sets first (within each
architecture) and wider unrollings first.

This doesn't make any difference right now, since all functions are
benchmarked; a follow-up change will make use of this by optionally
choosing the first valid function rather than testing all of them.

The Itanium raid6_intx{16,32} entries are also moved down to be near the
other raid6_intx entries for clarity.

Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Signed-off-by: Shaohua Li <shli@fb.com>