[Darwin] More specs TLC.

[Darwin] More specs TLC.

This strips out a few driver specs that are only specifying a default state.
Also warn on an option now ignored, and add some comments to the driver specs
section.

2019-07-19  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.h (DRIVER_SELF_SPECS): Ignore X and Mach specs which
	refer to default conditions.  Warn for the 'y' spec which is ignored
	by current linkers.

From-SVN: r273608