m68knommu: rename PT_OFF_VECTOR to PT_OFF_FORMATVEC.

In m68k/m68knommu assembly files, the same value is called sometimes
PT_OFF_VECTOR, but more frequently PT_OFF_FORMATVEC.  Standardize
name to PT_OFF_FORMATVEC.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>