Unify Book-E and AIM trap.c

Summary:
Book-E and AIM trap.c are almost identical, except for a few bits. This is step
1 in unifying them.

This also renumbers EXC_DEBUG, to not conflict with AIM vector numbers. Since
this is the only one thus far that is used in the switch statement in trap(),
it's the only one renumbered. If others get added to the switch, which conflict
with AIM numbers, they should also be renumbered.

Reviewers: PowerPC, marcel, nwhitehorn

Reviewed By: marcel

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D2215