sh: merge compiler flag -ffixed-r13

-ffixed-r13 is defined commonly for sh2, sh3, sh4.
Move it to arch/sh/config.mk

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>