target-arm: Restore IT bits when resuming after an exception

We were not correctly restoring the IT bits when resuming execution
after taking an unexpected exception in the middle of an IT block.
Fix this by tracking them along with PC changes and restoring in
gen_pc_load().

This fixes bug https://bugs.launchpad.net/qemu/+bug/581335

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
