target/arm: Use TRANS_FEAT for FMMLA

Being able to specify the feature predicate in TRANS_FEAT
makes it easier to split trans_FMMLA by element size,
which also happens to simplify the decode.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-79-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>