Replace Cortex A15 strlen implementation with cortex-strings

Benchmarks on a Nexus 10 have shown this implementation to be 5%-20%
faster (depending on input)

Change-Id: I4331a390c404a77b1e13909d713c4258a3776bca
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
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
libc: Fix memchr inclusion

Change-Id: I6f374f16b08e19851a375a4abad3e8235f8da59c