target/arm: Implement fp16 for Neon VRECPS

target/arm: Implement fp16 for Neon VRECPS

Convert the Neon VRECPS insn to using a gvec helper, and
use this to implement the fp16 case.

The phrasing of the new float32_recps_nf() is slightly different from
the old recps_f32() so that it parallels the f16 version; for f16 we
can't assume that flush-to-zero is always enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-34-peter.maydell@linaro.org