bionic: Add ARMv7 optimized string handling routines

Add ARMv7 - and especially Cortex-A9 - optimized string handling
routines based on the versions found in the cortex-strings library.

Change-Id: I497075bea5b82aef8989625bdec512029676fe7f
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>