kldconfig: Remove some code for compatibility with FreeBSD 4.x.

Verified using md5(1).
Always call vfp_discard() on thread death, not just when the VFP is
enabled.  In vfp_discard(), if the state in the VFP hardware belongs to
the thread which is dying, NULL out pcpu fpcurthread to indicate the
state currently in the hardware belongs to nobody.

Submitted by:	Juergen Weiss
Pointy hat to:	me
When a thread begins life it doesn't own the VFP hardware state on any cpu.
Make undefined exception entry MPSAFE.

The old code used static storage to preserve a couple registers while
setting up the trapframe for the main handler.  Doing so was the last
leftover crumbs from the days when a low-level debugger was hooked into
the exception entry code.

Now the exception entry sequence is essentially the same as for the
other exceptions, which still involves needlessly indirecting through
a function pointer which points to the same code on every platform.
Removing that indirection will be handled as a separate cleanup.

This work is based on an analysis by Juergen Weiss.
The arm exception entry points currently vector through a function pointer
to the actual handler routine.  All the pointers are static-intialized to
the only handlers available, and yet various platform-specific inits still
set those pointers (to the values they're already initialized to).  Begin
to drain the swamp by removing all the redundant external declarations and
runtime setting of the pointers that's scattered around various places.