string: ARMv8.5 MTE: Add MTE compatible version of strncmp.

Reading outside the range of the string is only allowed within 16 byte
aligned granules when MTE is enabled.

This implementation is based on string/aarch64/strncmp.S

Change the case when strings are are misaligned, align the pointers
down, and ignore bytes before the start of the string. Carry the part
that is not compared to the next comparison.

Testing done:
string/test/strncmp.c on big endian, little endian and with MTE support.
Booted nanodroid with MTE enabled.

Bechmarked on Pixel4.