The main reason that Darwin has been using DWARF2 only as debug is that
earlier debug linkers (dsymutil) did not support any extensions to this
so that the default "non-strict" mode used in GCC would cause tool errors.

There are two sources for dsymutil, those based off a closed source base
"dwarfutils" and those based off LLVM.

For dsymutil versions based off LLVM-7+ we can use up to DWARF-4, and for
versions based on dwarfutils 121+ we can use DWARF-3.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	* config/darwin-protos.h (enum darwin_external_toolchain): New.
	* config/darwin.cc (DSYMUTIL_VERSION): New.
	(darwin_override_options): Choose the default debug DWARF version
	depending on the configured dsymutil version.