[Darwin, PPC] Collate the system library spec into one expression.

[Darwin, PPC] Collate the system library spec into one expression.

There's no need to redefine this dependent on the target header (that only
works in the case that we have self-hosting which is less and less likely
for the older system versions).

Actually, what we need is for the correct library set to be used based
on the SDK(s) that can target the chosen system.

gcc/ChangeLog:

2019-07-10  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/darwin.h (LIB_SPEC): Collate this spec here.
	* config/rs6000/darwin7.h (LIB_SPEC): Remove.
	* config/rs6000/darwin8.h (LIB_SPEC): Remove.
	(DEF_MIN_OSX_VERSION): New.

From-SVN: r273369