kbuild: require all architectures to have arch/$(SRCARCH)/Kbuild

arch/$(SRCARCH)/Kbuild is useful for Makefile cleanups because you can
use the obj-y syntax.

Add an empty file if it is missing in arch/$(SRCARCH)/.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/Makefile b/Makefile
index b0bd0ce..4dcfe9c 100644
--- a/Makefile
+++ b/Makefile
@@ -658,7 +658,7 @@
 
 ifeq ($(KBUILD_EXTMOD),)
 # Objects we will link into vmlinux / subdirs we need to visit
-core-y		:= init/ usr/
+core-y		:= init/ usr/ arch/$(SRCARCH)/
 drivers-y	:= drivers/ sound/
 drivers-$(CONFIG_SAMPLES) += samples/
 drivers-$(CONFIG_NET) += net/