set kdb_why to "trap" when calling kdb_trap from trap_fatal

This will allow to hook a ddb script to "kdb.enter.trap" event.
Previously there was no specific name for this event, so it could only
be handled by either "kdb.enter.unknown" or "kdb.enter.default" hooks.
Both are very unspecific.

Having a specific event is useful because the fatal trap condition is
very similar to panic but it has an additional property that the current
stack frame is the frame where the trap occurred. So, both a register
dump and a stack bottom dump have additional information that can help
analyze the problem.

I have added the event only on architectures that have trap_fatal()
function defined. I haven't looked at other architectures. Their
maintainers can add support for the event later.

Sample script:
kdb.enter.trap=bt; show reg; x/aS $rsp,20; x/agx $rsp,20

Reviewed by: kib, jhb, markj
MFC after: 11 days
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D15093