target/arm: Handle FPCR.AH in negation steps in FCADD

target/arm: Handle FPCR.AH in negation steps in FCADD

The negation steps in FCADD must honour FPCR.AH's "don't change the
sign of a NaN" semantics.  Implement this by encoding FPCR.AH into
the SIMD data field passed to the helper and using that to decide
whether to negate the values.

The construction of neg_imag and neg_real were done to make it easy
to apply both in parallel with two simple logical operations.  This
changed with FPCR.AH, which is more complex than that. Switch to
an approach closer to the pseudocode, where we extract the rot
parameter from the SIMD data word and negate the appropriate
input value.

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