Summary:
Revert the initial FBT-with-KDB changes for trap_subr*.S, and instead use the
db_trap filter function to handle dtrace trap filtering. With this, the MMU is
enabled by the support code, simplifying the codepath altogether.
Test Plan: Tested on my G4 PowerBook
Reviewers: #powerpc, nwhitehorn
Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D1207
MFC after: 3 weeks