xtensa: syscalls: switch to generic syscalltbl.sh

Many architectures duplicate similar shell scripts.

This commit converts xtensa to use scripts/syscalltbl.sh.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210301153656.363839-2-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>