target/hppa: Set Float3NaNPropRule explicitly

target/hppa: Set Float3NaNPropRule explicitly

Set the Float3NaNPropRule explicitly for HPPA, and remove the
ifdef from pickNaNMulAdd().

HPPA is the only target that was using the default branch of the
ifdef ladder (other targets either do not use muladd or set
default_nan_mode), so we can remove the ifdef fallback entirely now
(allowing the "rule not set" case to fall into the default of the
switch statement and assert).

We add a TODO note that the HPPA rule is probably wrong; this is
not a behavioural change for this refactoring.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-26-peter.maydell@linaro.org