refine default dwarf settings on VxWorks

[Olivier Hainque <hainque at adacore dot com>]

Hello,

The Vxworks ports have specific default settings controlling the
dwarf output to accomodate the possible use of old non-gdb debuggers
provided with the VxWorks environment.

Those settings are hardcoded today, even though different versions
of VxWorks come with different constraints.

This patch just introduces per-vxworks-version variations on the
defaults we enforce.

We have been using a very minor variant of this patch in house for a couple
of years now. I verified a powerpc-wrs-vxworks and a powerpc-wrs-vxworksae
build works on mainline with the change, and that it produces the expected
effect.

Committing to mainline.

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

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

Attachment: vxworks-dwarf.diff
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