Fixed atomic access to flags in sys.execute.

This seems to fix the bug that caused Grbl to hang during some operations,
especially jogging.
This code isn't something I'm very familiar with. I don't know what the implications are by running this set of code. In general, I don't like code that is processor specific for Grbl, unless it is easy to work around for porting reasons. I'm not an expert at other processors, so I can't say whether or not all other micro controller can do something like this. But, I wouldn't be surprised if they could. 

Also, the cli() command: I'm not sure what the effect of this will be on everything else in the system. Is there any chance that the cli() command will clear or skip a serial interrupt, step pulse interrupt, or pin change interrupt?