target/arm: Implement FPCR.AH semantics for scalar FMIN/FMAX

target/arm: Implement FPCR.AH semantics for scalar FMIN/FMAX

When FPCR.AH == 1, floating point FMIN and FMAX have some odd special
cases:

 * comparing two zeroes (even of different sign) or comparing a NaN
   with anything always returns the second argument (possibly
   squashed to zero)
 * denormal outputs are not squashed to zero regardless of FZ or FZ16

Implement these semantics in new helper functions and select them at
translate time if FPCR.AH is 1 for the scalar FMAX and FMIN insns.
(We will convert the other FMAX and FMIN insns in subsequent
commits.)

Note that FMINNM and FMAXNM are not affected.

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