xtensa: syscalls: switch to generic syscallhdr.sh

Many architectures duplicate similar shell scripts.

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

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210301153656.363839-3-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>