target/s390x: meson: add target_user_arch

target/s390x: meson: add target_user_arch

the lack of target_user_arch makes it hard to fully leverage the
build system in order to separate user code from sysemu code.

Provide it, so that we can avoid the proliferation of #ifdef
in target code.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Signed-off-by: Cho, Yu-Chen <acho@suse.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210707105324.23400-2-acho@suse.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>