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

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

arch/alpha/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
+obj-y			+= kernel/ mm/
+obj-$(CONFIG_MATHEMU)	+= math-emu/

arch/alpha/Makefile

@@ -38,8 +38,6 @@ KBUILD_CFLAGS += $(cflags-y) -Wa,-mev6
 
 head-y := arch/alpha/kernel/head.o
 
-core-y				+= arch/alpha/kernel/ arch/alpha/mm/
-core-$(CONFIG_MATHEMU)		+= arch/alpha/math-emu/
 libs-y				+= arch/alpha/lib/
 
 # export what is needed by arch/alpha/boot/Makefile