ia64: move core-y in arch/ia64/Makefile to arch/ia64/Kbuild

Use obj-y to clean up Makefile.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/ia64/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
+obj-y				+= kernel/ mm/
+obj-$(CONFIG_IA64_SGI_UV)	+= uv/

arch/ia64/Makefile

@@ -47,8 +47,6 @@ KBUILD_CFLAGS += $(cflags-y)
 head-y := arch/ia64/kernel/head.o
 
 libs-y				+= arch/ia64/lib/
-core-y				+= arch/ia64/kernel/ arch/ia64/mm/
-core-$(CONFIG_IA64_SGI_UV)	+= arch/ia64/uv/
 
 drivers-y			+= arch/ia64/pci/ arch/ia64/hp/common/