i386: stop guessing the address of the trap frame in ddb backtrace.

Save the address of the trap frame in %ebp on kernel entry. This
automatically provides it in struct i386_frame.f_frame to unwinder.

While there, more accurately handle the terminating frames,

Reviewed by: avg, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D22321