target/arm: [tcg,a64] Port to translate_insn

Incrementally paves the way towards using the generic instruction translation
loop.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-Id: <150002510079.22386.10164419868911710218.stgit@frigg.lan>
[rth: Adjust for translate_insn interface change.]
Signed-off-by: Richard Henderson <rth@twiddle.net>