Replace the define_expand and two define_insns with a single
@macho_low_<mode> and update callers.

gcc/ChangeLog:

2019-09-29  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.c (gen_macho_low):Amend to include the mode
	argument.
	(machopic_indirect_data_reference): Amend gen_macho_low call
	to include mode argument
	* config/rs6000/rs6000.c (emit_move): Likewise. Amend a comment.
	* config/rs6000/darwin.md (@macho_low_<mode>): New, replaces
	the macho_high expander and two define_insn entries.

From-SVN: r276271