configure: Flip default of target_nptl

configure: Flip default of target_nptl

Most targets either (a) support threading or (b) don't have a
linux-user/bsd-user target, so make it default to 'yes', with those
targets that don't support threading having to specifically say so.

This is a mechanical no-behaviour-change commit; some of the
"no" entries it adds will be taken out in later patches.

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>