tcg: Introduce set/clear_helper_retaddr

tcg: Introduce set/clear_helper_retaddr

At present we have a potential error in that helper_retaddr contains
data for handle_cpu_signal, but we have not ensured that those stores
will be scheduled properly before the operation that may fault.

It might be that these races are not in practice observable, due to
our use of -fno-strict-aliasing, but better safe than sorry.

Adjust all of the setters of helper_retaddr.

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