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>