Darwin: Handle poly_int machine_modes.

Darwin: Handle poly_int machine_modes.

The common code that selects suitable sections for literals needs
to inspect the machine_mode.  For some sub-targets that might be
represented as a poly-int.

There was a workaround in place that allowed for cases where the poly
int had only one component.  This removes the workaround and handles
the cases where we care about the machine_mode size.

gcc/ChangeLog:

	* config/darwin.c (IN_TARGET_CODE): Remove.
	(darwin_mergeable_constant_section): Handle poly-int machine modes.
	(machopic_select_rtx_section): Likewise.