target/arm: Implement fp16 for Neon VFMA, VMFS

target/arm: Implement fp16 for Neon VFMA, VMFS

Convert the neon floating-point vector operations VFMA and VFMS
to use a gvec helper, and use this to implement the fp16 case.

This is the last use of do_3same_fp() so we can now delete
that function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-32-peter.maydell@linaro.org