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
6fc9f4dbc8