softfloat: make NO_SIGNALING_NANS runtime property

target/xtensa, the only user of NO_SIGNALING_NANS macro has FPU
implementations with and without the corresponding property. With
NO_SIGNALING_NANS being a macro they cannot be a part of the same QEMU
executable.
Replace macro with new property in float_status to allow cores with
different FPU implementations coexist.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: "Alex Bennée" <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

include/fpu/softfloat-types.h

@@ -165,8 +165,13 @@ typedef struct float_status {
     /* should denormalised inputs go to zero and set the input_denormal flag? */
     bool flush_inputs_to_zero;
     bool default_nan_mode;
-    /* not always used -- see snan_bit_is_one() in softfloat-specialize.h */
+    /*
+     * The flags below are not used on all specializations and may
+     * constant fold away (see snan_bit_is_one()/no_signalling_nans() in
+     * softfloat-specialize.inc.c)
+     */
     bool snan_bit_is_one;
+    bool no_signaling_nans;
 } float_status;
 
 #endif /* SOFTFLOAT_TYPES_H */