tcg: Promote tcg_out_{dup,dupi}_vec to backend interface

tcg: Promote tcg_out_{dup,dupi}_vec to backend interface

The i386 backend already has these functions, and the aarch64 backend
could easily split out one.  Nothing is done with these functions yet,
but this will aid register allocation of INDEX_op_dup_vec in a later patch.

Adjust the aarch64 tcg_out_dupi_vec signature to match the new interface.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>