fpu: Remove use_first_nan field from float_status

fpu: Remove use_first_nan field from float_status

The use_first_nan field in float_status was an xtensa-specific way to
select at runtime from two different NaN propagation rules.  Now that
xtensa is using the target-agnostic NaN propagation rule selection
that we've just added, we can remove use_first_nan, because there is
no longer any code that reads it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-27-peter.maydell@linaro.org