target/arm: Handle FPCR.AH in negation step in FMLS (indexed)

target/arm: Handle FPCR.AH in negation step in FMLS (indexed)

Handle the FPCR.AH "don't negate the sign of a NaN" semantics in FMLS
(indexed). We do this by creating 6 new helpers, which allow us to
do the negation either by XOR (for AH=0) or by muladd flags
(for AH=1).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Mostly from RTH's patch; error in index order into fns[][]
 fixed]
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>