Add int96 to multiint benchmark.

This was one idea to just cast to __int128_t as a poor man's int96.
Unfortunately, it seems too slow: ~15x for add, ~10x for multi and
3x for divide compared to __int128_t.

Change-Id: I06eb3fa3ac1edc2c174873a73a252a0165911b1c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2433
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins