target/arm/translate: make DISAS_UPDATE match declared semantics

DISAS_UPDATE should be used when the wider CPU state other than just
the PC has been updated and we should therefore exit the TCG runtime
and return to the main execution loop rather assuming DISAS_JUMP would
do that.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 20170713141928.25419-3-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
