powerpc: Optimized strncmp for power10

This patch is based on __strcmp_power10.

Improvements from __strncmp_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 38% better performance on average.
       - Minor performance regression is seen for few small sizes
	 and specific combination of alignments.

Signed-off-by: Amrita H S <amritahs@linux.ibm.com>
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>