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>

scripts/Makefile.asm-generic

@@ -12,6 +12,11 @@ all:
 src := $(subst /generated,,$(obj))
 -include $(src)/Kbuild
 
+# $(generic)/Kbuild lists mandatory-y. Exclude um since it is a special case.
+ifneq ($(SRCARCH),um)
+include $(generic)/Kbuild
+endif
+
 include scripts/Kbuild.include
 
 redundant := $(filter $(mandatory-y) $(generated-y), $(generic-y))