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/[e-mail]/T/#m7bc402e1e038f00ebcf2e92ed7fcb8a52fc1ea44

Signed-off-by: Masahiro Yamada <[e-mail]>
Tested-by: Jiri Slaby <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Tested-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/Kbuild

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

arch/um/Makefile

···
 # 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)