target/arm: Implement new VFP fp16 insn VMOVX

target/arm: Implement new VFP fp16 insn VMOVX

The fp16 extension includes a new instruction VMOVX, which copies the
upper 16 bits of a 32-bit source VFP register into the lower 16
bits of the destination and zeroes the high half of the destination.
Implement it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-21-peter.maydell@linaro.org