target/arm: Use macros instead of open-coding fp16 conversion helpers

target/arm: Use macros instead of open-coding fp16 conversion helpers

Now the VFP_CONV_FIX macros can handle fp16's distinction between the
width of the operation and the width of the type used to pass operands,
use the macros rather than the open-coded functions.

This creates an extra six helper functions, all of which we are going
to need for the AArch32 VFP fp16 instructions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-15-peter.maydell@linaro.org