Add support for dtrace:fbt on modules for PowerPC

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