fpu: Move m68k_denormal fmt flag into floatx80_behaviour

Currently we compile-time set an 'm68k_denormal' flag in the FloatFmt
for floatx80 for m68k.  This controls our handling of what the Intel
documentation calls a "pseudo-denormal": a value where the exponent
field is zero and the explicit integer bit is set.

For x86, the x87 FPU is supposed to accept a pseudo-denormal as
input, but never generate one on output.  For m68k, these values are
permitted on input and may be produced on output.

Replace the flag in the FloatFmt with a flag indicating whether the
float format has an explicit bit (which will be true for floatx80 for
all targets, and false for every other float type).  Then we can gate
the handling of these pseudo-denormals on the setting of a
floatx80_behaviour flag.

As far as I can see from the code we don't actually handle the
x86-mandated "accept on input but don't generate" behaviour, because
the handling in partsN(canonicalize) looked at fmt->m68k_denormal.
So I have added TODO comments to that effect.

This commit doesn't change any behaviour for any target.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250224111524.1101196-9-peter.maydell@linaro.org
Message-id: 20250217125055.160887-7-peter.maydell@linaro.org

target/m68k/cpu.c

@@ -139,7 +139,8 @@ static void m68k_cpu_reset_hold(Object *obj, ResetType type)
     set_floatx80_behaviour(floatx80_default_inf_int_bit_is_zero |
                            floatx80_pseudo_inf_valid |
                            floatx80_pseudo_nan_valid |
-                           floatx80_unnormal_valid,
+                           floatx80_unnormal_valid |
+                           floatx80_pseudo_denormal_valid,
                            &env->fp_status);
 
     nan = floatx80_default_nan(&env->fp_status);