sh: support CPU_J2 when compiler lacks -mj2

Signed-off-by: Rich Felker <dalias@libc.org>

arch/sh/Makefile

@@ -31,7 +31,7 @@ isa-y					:= $(isa-y)-up
 endif
 
 cflags-$(CONFIG_CPU_SH2)		:= $(call cc-option,-m2,)
-cflags-$(CONFIG_CPU_J2)			:= $(call cc-option,-mj2,)
+cflags-$(CONFIG_CPU_J2)			+= $(call cc-option,-mj2,)
 cflags-$(CONFIG_CPU_SH2A)		+= $(call cc-option,-m2a,) \
 					   $(call cc-option,-m2a-nofpu,) \
 					   $(call cc-option,-m4-nofpu,)