r315900:
  Cleanup structures related to VFP and/or mcontext_t.  - in mcontext_t,
  rename newer used 'union __vfp' to equaly sized 'mc_spare'.
    Space allocated by 'union __vfp' is too small and cannot hold full VFP
    context.
  - move structures defined in fp.h to more appropriate headers.  - remove
  all unused VFP structures.
r315973:
  Save VFP state on fork().  Update the copy of VFP state in PCB before it is
  cloned for new process.
r315974:
  Preserve VFP state across signal delivery.