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.

Only compile tested for all affected architectures. There can be bugs
resulting from my poor understanding of architecture specific details.

Reported by: jhb
Reviewed by: jhb, eadler
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D15431