softfloat: add xtensa specialization for pickNaNMulAdd

softfloat: add xtensa specialization for pickNaNMulAdd

pickNaNMulAdd logic on Xtensa is to apply pickNaN to the inputs of the
expression (a * b) + c. However if default NaN is produces as a result
of (a * b) calculation it is not considered when c is NaN.
So with two pickNaN variants there must be two pickNaNMulAdd variants.
In addition the invalid flag is always set when (a * b) produces NaN.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: "Alex Bennée" <alex.bennee@linaro.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>