m68k: Pass -D options to KBUILD_CPPFLAGS instead of KBUILD_{A,C}FLAGS

Precisely, -D is a preprocessor option.

KBUILD_CPPFLAGS is passed for compiling .c and .S files too.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200526123810.301667-4-masahiroy@kernel.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/Makefile

@@ -70,9 +70,8 @@ ifdef CONFIG_MMU
 KBUILD_CFLAGS += -fno-strength-reduce -ffixed-a2
 else
 # we can use a m68k-linux-gcc toolchain with these in place
-KBUILD_CFLAGS += -DUTS_SYSNAME=\"uClinux\"
-KBUILD_CFLAGS += -D__uClinux__
-KBUILD_AFLAGS += -D__uClinux__
+KBUILD_CPPFLAGS += -DUTS_SYSNAME=\"uClinux\"
+KBUILD_CPPFLAGS += -D__uClinux__
 endif
 
 KBUILD_LDFLAGS := -m m68kelf