[Darwin, PPC, Mode Iterators 0/n] Make iterators visible to darwin.md.

As a clean-up, we want to be able to use mode iterators in darwin.md.
This patch moves the include point for the Darwin include until after
the definition of the mode iterators and attrs.  No functional change
intended.

gcc/ChangeLog:

2019-09-24  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/rs6000.md: Move darwin.md include until
	after the definition of the mode iterators.

From-SVN: r276106