Import memchr implementation from cortex-strings for Cortex A15

Benchmarks on a Nexus 10 have shown the cortex-strings implementation
of memchr to be 60%-70% faster (depending on input)

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

Change-Id: I72b119905234570e54068375e5c3593d0647c67c

Conflicts:
	libc/arch-arm/generic/generic.mk