target/arm: Use tcg_gen_gvec_bitsel for trans_SEL_pppp

The gvec operation was added after the initial implementation
of the SEL instruction and was missed in the conversion.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200815013145.539409-8-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>