target/arm: Add helpers for FMLAL

target/arm: Add helpers for FMLAL

Note that float16_to_float32 rightly squashes SNaN to QNaN.
But of course pickNaNMulAdd, for ARM, selects SNaNs first.
So we have to preserve SNaN long enough for the correct NaN
to be selected.  Thus float16_to_float32_by_bits.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190219222952.22183-2-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>