move cflags setting to arch/<arch>/Makefile

Move Zephyr specific CFLAGS that depend on config options to
the Makefile where they can better be managed among other cflag
options.

Change-Id: Ia79a2f2def4f51857f6d661aa78e9fb7eb7a5e22
Signed-off-by: Anas Nashif <anas.nashif@intel.com>