Use -mcpu=cortex-a15 for gcc arch dependent compilation.

If TARGET_CPU_VARIANT=cortex-a15 is specified in the BoardConfig.mk file.

Bug: 7961327

cherry-picked from internal master.

Change-Id: I6679fa7fc277f263ee2af4de2051ed0d4366a263

Conflicts:
	core/combo/arch/arm/armv7-a-neon.mk
	core/llvm_config.mk