Darwin : Adjust defaults for the linker.

Ideally, the linker will be queried for its version and that will be
used to determine capabilities that cannot be discovered from
reasonable configuration testing.

When building cross tools, this might not be possible, and we have
strategies for providing useful defaults.  These are adjusted here to
refect current choices.

gcc/ChangeLog:

	* config/darwin.h (MIN_LD64_NO_COAL_SECTS): Adjust.
	Amend handling for LD64_VERSION fallback defaults.