Revert "build: Relax Cortex CFLAGS"

This reverts commit 58dc899c94a4d501a992ad432975285b7c154464

we already have what this does. it also reverts a commit from linaro.
Avoid mtuning an empty TARGET_ARCH_VARIANT_CPU

Change-Id: I385fa7a5bee2734eb95ab92fc3a36589c48bb0ef