xtensa: do not build variants directory

None of arch/xtensa/variants/*/ has Makefile, so 'buildvar' is always
empty.

Perhaps, downstream variant code might be dropped in, but given the
fact that none of upstream variants builds anything in their variant
directory, I doubt this is needed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210811163731.186125-2-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>