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.

MFC after: 2 weeks