softfloat: pass float_status pointer to pickNaN

Pass float_status structure pointer to the pickNaN so that
machine-specific settings are available to NaN selection code.
Add use_first_nan property to float_status and use it in Xtensa-specific
pickNaN.

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

include/fpu/softfloat-types.h

@@ -171,6 +171,7 @@ typedef struct float_status {
      * softfloat-specialize.inc.c)
      */
     bool snan_bit_is_one;
+    bool use_first_nan;
     bool no_signaling_nans;
 } float_status;