build: Enable Bionic optimizations on Cortex platforms

 * Enable the new NEON optimizations in Bionic when
   TARGET_ARCH_VARIANT_CPU is set.
 * NEON memcpy() optimization is disabled on A15 due to a regression.

Change-Id: I4862695812a70b75bdabc4398cb592157f3913c6
build: move bionic optimizations to NEON makefile

Change-Id: Idd495ddbb49f67fef5496456803263f8a23643c5
build: Relax Cortex CFLAGS

 * Always set march=armv7-a
 * Use mtune instead of mcpu. Bad code is generated on A15 otherwise.

Change-Id: Idf8ced8eb0e9d3d4676ca8ec69bf56aed5533bf4