target/arm: Implement vector float32 to bfloat16 conversion

target/arm: Implement vector float32 to bfloat16 conversion

This is BFCVT{N,T} for both AArch64 AdvSIMD and SVE,
and VCVT.BF16.F32 for AArch32 NEON.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210525225817.400336-5-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>