translate-all: exit cpu_restore_state early if translating

The translation code uses cpu_ld*_code which can trigger a tlb_fill
which if it fails will erroneously attempts a fault resolution. This
never works during translation as the TB being generated hasn't been
added yet. The target should have checked retaddr before calling
cpu_restore_state but for those that have yet to be fixed we do it
here to avoid a recursive tb_lock() under MTTCG's new locking regime.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
