[Darwin] Some TLC for older Darwin versions.

[Darwin] Some TLC for older Darwin versions.

The library handling and some of the options for creating the crts for
the older PPC Darwin versions had bit-rotted somewhat.  This adjusts the
build criteria for the crts to avoid newer ld64 versions warnings about
mismatches in build and object versions.

Added to some of the comments that it's documented why the specs are as
they are.

2019-07-03  Iain Sandoe  <iain@sandoe.co.uk>

gcc/

	* config/darwin.h (REAL_LIBGCC_SPEC): Adjust for earlier Darwin.
	(STARTFILE_SPEC): Split crt3 into a separate spec.
	(DARWIN_EXTRA_SPECS): Add crt2 and crt3 spec.
	(DARWIN_CRT2_SPEC): New.
	(DARWIN_CRT3_SPEC): New.
	(MIN_LD64_OMIT_STUBS): Revise to 62.1.
	* config/rs6000/darwin.h (DARWIN_CRT2_SPEC): Revise conditions.
	(DARWIN_CRT3_SPEC): New.

libgcc/

2019-07-03  Iain Sandoe  <iain@sandoe.co.uk>

	* config.host (powerpc-*-darwin*,powerpc64-*-darwin*): Revise crt
	list.
	* config/rs6000/t-darwin: Build crt3_2 for older systems.  Revise
	mmacosx-version-min for crts to run across all system versions.
	* config/rs6000/t-darwin64 (LIB2ADD): Remove.
	* config/t-darwin: Revise mmacosx-version-min for crts to run across
	system versions >= 10.4.

From-SVN: r273016