arm64: lib: Implement optimized memcmp routine

This patch, based on Linaro's Cortex Strings library, adds
an assembly optimized memcmp() function.

Signed-off-by: Zhichang Yuan <zhichang.yuan@linaro.org>
Signed-off-by: Deepak Saxena <dsaxena@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: d875c9b3724083cd2629cd8507e424cd3716cd28
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>