Increase loop unrolling for skein hashes

This patch was inspired by an opposite change made to shrink the code
for the boot loader.

On my i7-4770, it increases the skein1024 speed from 470 to 550 MB/s

Reviewed by: sbruno
MFC after: 1 month
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D7824