Darwin, opts processing - Initial tidy.

This change adds RejectNegative in a number of places it was needed,
reorders the opts into two groups (one general and one driver-only).
We also add a minimal description to each opt, and note some that
are now obsolete.  The only functional change is to permit
-mtarget-linker= as an alias of -mtarget-linker.

gcc/

2019-06-14  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.opt: Add RejectNegative where needed, reorder
	and add minimal functional descriptions.

From-SVN: r272308

gcc/ChangeLog

+2019-06-14  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* config/darwin.opt: Add RejectNegative where needed, reorder
+	and add minimal functional descriptions.
+
 2019-06-14  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR rtl-optimization/90765