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

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

Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-Id: <150002606914.22386.15524101311003685068.stgit@frigg.lan>
[rth: Move tb->size computation and use that result.]
Signed-off-by: Richard Henderson <rth@twiddle.net>