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

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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@335557 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f