powerpc: Optimized strcmp for power10

This patch is based on __strcmp_power9 and __strlen_power10.

Improvements from __strcmp_power9:

    1. Uses new POWER10 instructions
       - This code uses lxvp to decrease contention on load
         by loading 32 bytes per instruction.

    2. Performance implication
       - This version has around 30% better performance on average.
       - Performance regression is seen for a specific combination
         of sizes and alignments. Some of them is observed without
         changes also, while rest may be induced by the patch.

Signed-off-by: Amrita H S <amritahs@linux.vnet.ibm.com>
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>