arm64: Import latest version of Cortex Strings' strncmp

Import the latest version of the former Cortex Strings - now
Arm Optimized Routines - strncmp function based on the upstream
code of string/aarch64/strncmp.S at commit e823e3a from
https://github.com/ARM-software/optimized-routines

Note that for simplicity Arm have chosen to contribute this code
to Linux under GPLv2 rather than the original MIT license.
Signed-off-by: Sam Tebbs <sam.tebbs@arm.com>
[ rm: update attribution and commit message ]
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/26110bee02ad360596c9a7536af7eaaf6890d0e8.1622128527.git.robin.murphy@arm.comSigned-off-by: Will Deacon <will@kernel.org>