target/i386: Set 2-NaN propagation rule explicitly

target/i386: Set 2-NaN propagation rule explicitly

Set the NaN propagation rule explicitly for the float_status words
used in the x86 target.

This is a no-behaviour-change commit, so we retain the existing
behaviour of using the x87-style "prefer QNaN over SNaN, then prefer
the NaN with the larger significand" for MMX and SSE.  This is
however not the documented hardware behaviour, so we leave a TODO
note about what we should be doing instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241025141254.2141506-16-peter.maydell@linaro.org