ARM: refactor compiler options in config.mk

Every ARM cpu config.mk (arch/arm/cpu/{CPUDIR}/config.mk) defines:

PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float

So, this patch moves the common compiler options to arch/arm/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>