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

This version of memcpy, taken from cortex-strings revision 103, improves
performance over the existing implementation on the majority of copies.

Benchmarks and tests for this routine can be found as part of the
cortex-strings project on Launchpad:

https://launchpad.net/cortex-strings

Change-Id: I0b5f76fadb8cca069a6847456637061ca067e5b7
Signed-off-by: Will Newton <will.newton@linaro.org>

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