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

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: <150002558503.22386.1149037590886263349.stgit@frigg.lan>
Signed-off-by: Richard Henderson <rth@twiddle.net>