Prevent traps after rsp modification

If trap happens after rsp is modified but before it is sandboxed, Win exception
handler fails to run. This applies to breakpoints and hardware-supported single
step.

We fix these by:
- use software single step to step both rsp modification and rsp sandboxing at once
- moving breakpoints from after rsp modification to after rsp sandboxing

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2903

Review URL: https://chromiumcodereview.appspot.com/10832095
1 file changed