fpu: Make floatx80 invalid encoding settable at runtime

Because floatx80 has an explicit integer bit, this permits some
odd encodings where the integer bit is not set correctly for the
floating point value type. In In Intel terminology the
 categories are:
  exp == 0, int = 0, mantissa == 0 : zeroes
  exp == 0, int = 0, mantissa != 0 : denormals
  exp == 0, int = 1 : pseudo-denormals
  0 < exp < 0x7fff, int = 0 : unnormals
  0 < exp < 0x7fff, int = 1 : normals
  exp == 0x7fff, int = 0, mantissa == 0 : pseudo-infinities
  exp == 0x7fff, int = 1, mantissa == 0 : infinities
  exp == 0x7fff, int = 0, mantissa != 0 : pseudo-NaNs
  exp == 0x7fff, int = 1, mantissa == 0 : NaNs

The usual IEEE cases of zero, denormal, normal, inf and NaN are always valid.
x87 permits as input also pseudo-denormals.
m68k permits all those and also pseudo-infinities, pseudo-NaNs and unnormals.

Currently we have an ifdef in floatx80_invalid_encoding() to select
the x86 vs m68k behaviour.  Add new floatx80_behaviour flags to
select whether pseudo-NaN and unnormal are valid, and use these
(plus the existing pseudo_inf_valid flag) to decide whether these
encodings are invalid at runtime.

We leave pseudo-denormals as always-valid, since both x86 and m68k
accept them.

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: 20250224111524.1101196-8-peter.maydell@linaro.org
Message-id: 20250217125055.160887-6-peter.maydell@linaro.org

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