target/arm: Implement VFP fp16 for VABS, VNEG, VSQRT

target/arm: Implement VFP fp16 for VABS, VNEG, VSQRT

Implement VFP fp16 for VABS, VNEG and VSQRT. This is all
the fp16 insns that use the DO_VFP_2OP macro, because there
is no fp16 version of VMOV_reg.

Notes:
 * the gen_helper_vfp_negh already exists as we needed to create
   it for the fp16 multiply-add insns
 * as usual we need to use the f16 version of the fp_status;
   this is only relevant for VSQRT

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-9-peter.maydell@linaro.org