um: use obj-y to descend into arch/um/*/

The single build rule does not work with the core-y syntax. [1]

Use the standard obj-y syntax.

[1]: https://lore.kernel.org/linux-kbuild/d57ba55f-20a3-b836-783d-b49c8a161b6e@kernel.org/T/#m7bc402e1e038f00ebcf2e92ed7fcb8a52fc1ea44Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
+
+obj-y += kernel/ drivers/ os-Linux/

arch/um/Makefile

@@ -22,10 +22,6 @@ ARCH_DIR := arch/um
 # features.
 SHELL := /bin/bash
 
-core-y			+= $(ARCH_DIR)/kernel/		\
-			   $(ARCH_DIR)/drivers/		\
-			   $(ARCH_DIR)/os-Linux/
-
 MODE_INCLUDE	+= -I$(srctree)/$(ARCH_DIR)/include/shared/skas
 
 HEADER_ARCH 	:= $(SUBARCH)