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

Use obj-y to clean up Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210811163731.186125-4-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>