AR: tcontrol.mie applies to all traps, not just breakpoints

Reverts #723, which would result in mepc etc. being clobbered if a
breakpoint trigger would fire in the most privileged mode.

Specifically, consider:
1. icount is set so that it'll trigger on the first instruction of an exception handler.
2. Code takes an exception, saving mepc etc.
3. Now icount fires causing a breakpoint exception, clobbering mepc.