configure: Enable threading on all ppc and mips linux-user targets

configure: Enable threading on all ppc and mips linux-user targets

Though threading (target_nptl) was enabled on the base PPC and MIPS
targets, it wasn't enabled for the variants mipsn32, mipsn32el,
mips64, mips64el, ppc64 or ppc46abi32 (probably an oversight).
Enable threading consistently on all these targets.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>