Replace memcpy with the cortex-strings implementation

Benchmarks on a Nexus 10 have shown this implementation to be
up to 12% faster.

Change-Id: I212df99357b338221779cfba118ebfff441c9457
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	libc/arch-arm/cortex-a15/bionic/memcpy.S