Protect trap polling against interrupts in Ensemble's OS

[endorpersand]

Section 9.5 of the textbook mentions that data corruption can occur if an interrupt happens during a polling trap. The textbook resolves this by introducing a notion of disabling/enabling interrupts.

Currently, Ensemble does not do this. However, as far as I know, Complx and LC3Tools v3.1.0 also did not do this. It is an extremely minor and pedantic problem which will almost certainly never occur, but for correctness, it probably should be implemented.

The textbook's discussion in 9.5 introduces a notion of PSR[14] as a system-wide interrupt toggle (and proceeds to never define this again–it's not even in the LC-3 specification). This can probably instead be supplemented with setting the priority of the PSR to 7.