soc: arm: Unify setting of Cortex-M specifc compiler flags

Moved setting of specific Cortex-M compiler flags in each SoC directory
unify setting them in the arch/arm/soc Makefile.

Add flags for Cortex M0, M0+, M1, M3, M4, and M7.  However only
CONFIG_CPU_CORTEX_M3 and CONFIG_CPU_CORTEX_M4 are supported at this time

As part of this change converted Kbuild files for some SoCs into
Makefiles as the Makefiles would be empty otherwise.

Change-Id: Ie4e0178b141ca761ec482a610ae50e94fe58070f
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>