MFC r333667: followup to r332730/r332752: set kdb_why to "trap" for fatal traps

This change updates arm, arm64 and mips achitectures. Additionally, it
removes redundant checks for kdb_active where it already results in
kdb_reenter() and adds kdb_reenter() calls where they were missing.

Some architectures check the return value of kdb_trap(), but some don't.
I haven't changed any of that.

Some trap handling routines have a return code. I am not sure if I
provided correct ones for returns after kdb_reenter(). kdb_reenter
should never return unless kdb_jmpbufp is NULL for some reason.

MFC r266826, r266827
Move some duplicated hook definitions from machine-dependent files to
kern_dtrace.c.

MFC r257417: Remove references to an unused fasttrap probe hook

MFC r256939

MFP4:
Change 221767 by rwatson@rwatson_zenith_cl_cam_ac_uk on 2013/02/05 14:18:53

When printing out information on a TLB MOD exception for a user
process (e.g., an attempt to write to a read-only page), report
it as a "write" in the console message, rather than "unknown".

Change 221768 by rwatson@rwatson_zenith_cl_cam_ac_uk on 2013/02/05 14:28:00

Fix post-compile but pre-commit typo in last changeset.

Sponsored by: DARPA/AFRL

MFC r266826, r266827
Move some duplicated hook definitions from machine-dependent files to
kern_dtrace.c.

MFC r257417: Remove references to an unused fasttrap probe hook

MFC r256939

MFP4:
Change 221767 by rwatson@rwatson_zenith_cl_cam_ac_uk on 2013/02/05 14:18:53

When printing out information on a TLB MOD exception for a user
process (e.g., an attempt to write to a read-only page), report
it as a "write" in the console message, rather than "unknown".

Change 221768 by rwatson@rwatson_zenith_cl_cam_ac_uk on 2013/02/05 14:28:00

Fix post-compile but pre-commit typo in last changeset.

Sponsored by: DARPA/AFRL