softfloat: abstract out target-specific NaN propagation rules IEEE754 doesn't specify precisely what NaN should be returned as the result of an operation on two input NaNs. This is therefore target-specific. Abstract out the code in propagateFloat*NaN() which was implementing the x87 propagation rules, so that it can be easily replaced on a per-target basis. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>