target/arm: Implement FPCR.AH handling for scalar FABS and FABD

target/arm: Implement FPCR.AH handling for scalar FABS and FABD

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

Implement these semantics for scalar FABS and FABD.  This change also
affects all other instructions whose psuedocode calls FPAbs(); we
will extend the change to those instructions in following commits.

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