target/arm: [tcg] Port to disas_log

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

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Benneé <alex.benee@linaro.org>
Message-Id: <150002582711.22386.191527630537864599.stgit@frigg.lan>
[rth: Move tb->size computation and use that result.]
Signed-off-by: Richard Henderson <rth@twiddle.net>