Add RAID-Z routines for SSE2 instruction set, in x86_64 mode.

Add RAID-Z routines for SSE2 instruction set, in x86_64 mode.

The patch covers low-end and older x86 CPUs.  Parity generation is
equivalent to SSSE3 implementation, but reconstruction is somewhat
slower.  Previous 'sse' implementation is renamed to 'ssse3' to
indicate highest instruction set used.

Benchmark results:
scalar_rec_p                    4    720476442
scalar_rec_q                    4    187462804
scalar_rec_r                    4    138996096
scalar_rec_pq                   4    140834951
scalar_rec_pr                   4    129332035
scalar_rec_qr                   4    81619194
scalar_rec_pqr                  4    53376668

sse2_rec_p                      4    2427757064
sse2_rec_q                      4    747120861
sse2_rec_r                      4    499871637
sse2_rec_pq                     4    522403710
sse2_rec_pr                     4    464632780
sse2_rec_qr                     4    319124434
sse2_rec_pqr                    4    205794190

ssse3_rec_p                     4    2519939444
ssse3_rec_q                     4    1003019289
ssse3_rec_r                     4    616428767
ssse3_rec_pq                    4    706326396
ssse3_rec_pr                    4    570493618
ssse3_rec_qr                    4    400185250
ssse3_rec_pqr                   4    377541245

original_rec_p                  4    691658568
original_rec_q                  4    195510948
original_rec_r                  4    26075538
original_rec_pq                 4    103087368
original_rec_pr                 4    15767058
original_rec_qr                 4    15513175
original_rec_pqr                4    10746357

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4783