Downgrade DWARF_VERSION_DEFAULT to 3 for VxWorks >= 7

Using 4 as the DWARF_VERSION_DEFAULT value for VxWorks observably
still hangs recent system debuggers on tbreak for some contructs,
and downgrading to 3 improves the situation.

2022-03-06  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/vxworks.h (DWARF_VERSION_DEFAULT): Adjust from
	4 to 3 for VxWorks >= 7.