target/arm: Use TRANS_FEAT for RAX1

The decode for RAX1 sets esz to MO_8, because that's what
we use by default for "no esz present".  We changed that
to MO_64 during translation because it is more logical for
the operation.  However, the esz argument to gen_gvec_rax1
is unused and forces MO_64 within that function, so there
is no need to do it here as well.

Simplify to use gen_gvec_fn_arg_zzz and TRANS_FEAT.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-36-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>