Add cortex-a53.a57 target for 32 bit big/little.

The routines optimized for cortex-a7 and cortex-a53 cause performance
drops on cortex-a57. Instead create a target that is the middle ground
that works relatively well on either core.

Change-Id: Ie2b6cc9a59a01c7b30602ee368b2b90f5e886289