target/arm: Use TRANS_FEAT for FRECPE, FRSQRTE

Rename do_zz_fp to gen_gvec_fpst_arg_zz, and move up.
Split out gen_gvec_fpst_zz as a helper while we're at it.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-92-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>