[Darwin, PPC] Clean up symbol stubs code.

[Darwin, PPC] Clean up symbol stubs code.

Remove dead code for the the TARGET_LINK_STACK which is not
applicable to Darwin. Use MACHOPIC_PURE instead of a hard-wired
PIC level to determine the stub kind.

Merge common code blocks.

gcc/ChangeLog:

2019-09-22  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/rs6000.c (machopic_output_stub): Remove dead
	code.  Merge code blocks with common conditionals. Use declared
	macro instead of a magic number for PIC level.

From-SVN: r276030