powerpc: Fix performance issues of strcmp power10

powerpc: Fix performance issues of strcmp power10

Current implementation of strcmp for power10 has
performance regression for multiple small sizes
and alignment combination.

Most of these performance issues are fixed by this
patch. The compare loop is unrolled and page crosses
of unrolled loop is handled.

Thanks to Paul E. Murphy for helping in fixing the
performance issues.

Signed-off-by: Amrita H S <amritahs@linux.vnet.ibm.com>
Co-Authored-By: Paul E. Murphy <murphyp@linux.ibm.com>
Reviewed-by: Rajalakshmi Srinivasaraghavan <rajis@linux.ibm.com>