[Darwin, PPC, Mode Iterators 1/n] Use mode iterators in picbase patterns.

This switches the picbase load and reload patterns to use the 'P' mode
iterator instead of writing an SI and DI pattern for each.

gcc/ChangeLog:

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

	* config/rs6000/rs6000.md (load_macho_picbase_<mode>): New, using
	the 'P' mode iterator, replacing the (removed) SI and DI variants.
	(reload_macho_picbase_<mode>): Likewise.

From-SVN: r276107