ARM: merge commonly-defined PLATFORM_RELFLAGS

Before this commit, all arch/arm/cpu/${CPU}/config.mk except ARMv8
had the same option:
$(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))

This commit moves it into arch/arm/config.mk.

If the compiler does not support the option,
it is ignored by $(call cc-option,...).
So this commit gives no harm to ARMv8.

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