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

Conflicts:
	core/combo/arch/arm/armv7-a.mk
build: move bionic optimizations to NEON makefile

Change-Id: Idd495ddbb49f67fef5496456803263f8a23643c5

Conflicts:
	core/combo/arch/arm/armv7-a-neon.mk
build: Filter out cortex-a15 CFLAG for clang

 * Clang doesn't support this target yet, so filter it out.

Change-Id: I94aeae7562ff5377061ef17e1d762533f9ca55f7