vxworks.c (vxworks_override_options): Pick default dwarf version from macro value, VXWORKS_DWARF_VERSION_DEFAULT.

2017-11-22  Olivier Hainque  <hainque@adacore.com>

       * config/vxworks.c (vxworks_override_options): Pick default
	dwarf version from macro value, VXWORKS_DWARF_VERSION_DEFAULT.
       * config/vxworks.h: Define VXWORKS_DWARF_VERSION_DEFAULT and
       DWARF_GNAT_ENCODINGS_DEFAULT.
       * config/vxworksae.h: Likewise.

From-SVN: r255069