Darwin,debug : Switch to DWARF 3 or 4 when dsymutil supports it.

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.