Preserve VFP state across signal delivery.

Preserve VFP state across signal delivery.

We don't have enouch space to store full VFP context within mcontext
stucture. Due to this:
 - follow i386/amd64 way and store VFP state outside of the mcontext_t
   but point to it. Use the size of VFP state structure as an 'magic'
   indicator of the saved VFP state presence.
 - teach set_mcontext() about this external storage.
 - for signal delivery, store VFP state to expanded 'struct sigframe'.

Submited by:	Andrew Gierth (initial version)
PR:		217611
MFC after:	2 weeks