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>

include/uapi/asm-generic/Kbuild

@@ -1,6 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0
 #
 # Headers that are mandatory in usr/include/asm/
-#
+# (This file is not included when SRCARCH=um since UML does not support UAPI.)
+
 mandatory-y += auxvec.h
 mandatory-y += bitsperlong.h
 mandatory-y += bpf_perf_event.h