kbuild: force all architectures except um to include mandatory-y

Currently, every arch/*/include/uapi/asm/Kbuild explicitly includes
the common Kbuild.asm file. Factor out the duplicated include directives
to scripts/Makefile.asm-generic so that no architecture would opt out
of the mandatory-y mechanism.

um is not forced to include mandatory-y since it is a very exceptional
case which does not support UAPI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index 08a01ad..4f45ae6 100644
--- a/Makefile
+++ b/Makefile
@@ -1098,9 +1098,11 @@
 
 PHONY += asm-generic uapi-asm-generic
 asm-generic: uapi-asm-generic
-	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/asm
+	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/asm \
+	generic=include/asm-generic
 uapi-asm-generic:
-	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/uapi/asm
+	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/uapi/asm \
+	generic=include/uapi/asm-generic
 
 PHONY += prepare-objtool
 prepare-objtool: $(objtool_target)