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

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

arch/hexagon/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
+obj-y += kernel/ mm/ lib/

arch/hexagon/Makefile

@@ -34,7 +34,3 @@ KBUILD_CFLAGS += -ffixed-$(TIR_NAME) -DTHREADINFO_REG=$(TIR_NAME) -D__linux__
 KBUILD_AFLAGS += -DTHREADINFO_REG=$(TIR_NAME)
 
 head-y := arch/hexagon/kernel/head.o
-
-core-y += arch/hexagon/kernel/ \
-	arch/hexagon/mm/ \
-	arch/hexagon/lib/