configure: Don't say target_nptl="no" if there is no linux-user target
For architectures with no linux-user target, don't claim no NPTL
support. This has no behavioural change, but it means that we
won't accidentally add a new linux-user target without threading
support in future (because attempting to do so would be a compile
failure rather than a silent lack of support).
Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <rth@twiddle.net> Signed-off-by: Riku Voipio <riku.voipio@linaro.org>