[Darwin, PPC] Install the same headers as other sub-targets.

[Darwin, PPC] Install the same headers as other sub-targets.

This is primarily in order to improve testsuite coverage, we might elect
to prune the list at some point.

gcc/

2019-11-01  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline
	2019-06-28  Iain Sandoe  <iain@sandoe.co.uk>

	* config.gcc (powerpc-*-darwin*, powerpc64-*-darwin*): Remove
	override on extra_headers.

From-SVN: r277720