target/arm: Implement FPCR.AH handling of negation of NaN

target/arm: Implement FPCR.AH handling of negation of NaN

FPCR.AH == 1 mandates that negation of a NaN value should not flip
its sign bit.  This means we can no longer use gen_vfp_neg*()
everywhere but must instead generate slightly more complex code when
FPCR.AH is set.

Make this change for the scalar FNEG and for those places in
translate-a64.c which were previously directly calling
gen_vfp_neg*().

This change in semantics also affects any other instruction whose
pseudocode calls FPNeg(); in following commits we extend this
change to the other affected instructions.

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