Add x86_64 ssse3, avx, avx2 optimized sha256

The sha256-{ssse3,avx,avx2}-asm.S files are copied from linux-3.14.

Currently, we only support optimization in kernel space. User programs will
use generic code.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Add a simple unit test for sha256

Run some simple tests on predefined strings when ZFS_DEBUG is on.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>