fpu: Add float_class_denormal

fpu: Add float_class_denormal

Currently in softfloat we canonicalize input denormals and so the
code that implements floating point operations does not need to care
whether the input value was originally normal or denormal.  However,
both x86 and Arm FEAT_AFP require that an exception flag is set if:
 * an input is denormal
 * that input is not squashed to zero
 * that input is actually used in the calculation (e.g. we
   did not find the other input was a NaN)

So we need to track that the input was a non-squashed denormal.  To
do this we add a new value to the FloatClass enum.  In this commit we
add the value and adjust the code everywhere that looks at FloatClass
values so that the new float_class_denormal behaves identically to
float_class_normal.  We will add the code that does the "raise a new
float exception flag if an input was an unsquashed denormal and we
used it" in a subsequent commit.

There should be no behavioural change in this commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>