Do not duplicate arch properties when cpu variant == arch variant

build/make/board/generic_mips/BoardConfig.mk sets
TARGET_CPU_VARIANT := TARGET_ARCH_VARIANT, which can cause the same
variant-specific properties to be applied twice.

Test: lunch aosp_mips-eng && mmma -j external/skia TARGET_ARCH_VARIANT=mips32r2dspr2-fp
Change-Id: Ie4abe804f8ce898bf89664ae749b9bc8ac3e649f