Introduce DWARF_VERSION_DEFAULT and redefine for VxWorks

Introduce DWARF_VERSION_DEFAULT and redefine for VxWorks

This change introduces a target overridable macro to replace
the hardcoded value used to initialize dwarf_version from common.opt.

The main advantage compared to special code in a target
override_options hook is that redefinitions by target config files
are visible by both the compiler proper and by the driver, which
might refer to dwarf_version in ASM_DEBUG_SPECs and friends.

This is useful at least on VxWorks, where we usually need to
default to dwarf 4 or even 2 to accommodate non-gdb debuggers
provided by the environment, including for assembly sources
used in libgcc for some ports (witnessed with lse.S on aarch64).

2022-02-28  Olivier Hainque  <hainque@adacore.com>

gcc/
	* defaults.h (DWARF_DEFAULT_VERSION): Define if not
	defined already.
	* common.opt (gdwarf-): Use it.
	* doc/tm.texi.in (DWARF_DEFAULT_VERSION): Document.
	* doc/tm.texi: Update accordingly.

	* config/vxworks.h (DWARF_DEFAULT_VERSION): Redefine.
	* config/vxworks.cc: Remove code setting dwarf_version, now
	handled by the DWARF_DEFAULT_VERSION redefinition.