arm64: lib: Implement optimized memset routine

This patch, based on Linaro's Cortex Strings library, improves
the performance of the assembly optimized memset() 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: b29a51fe0e0be63157d8661666be8bbfd8f0c5d7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>