[Darwin] Amend section for constants with reloactions.

[Darwin] Amend section for constants with reloactions.

Darwin's linker doesn't like text section relocations (they require special
enabling).  At least, the Fortran FE seems to generate cases where the
initialiser for a pointer constant can need a relocation.  We can handle
this by special-casing SECCAT_RODATA when the relocation is present by
placing the constant in the .const_data section.

gcc/ChangeLog:

2019-10-05  Iain Sandoe  <iain@sandoe.co.uk>

	PR target/59888
	* config/darwin.c (darwin_rodata_section): Add relocation flag,
	choose const_data section for constants with relocations.
	(machopic_select_section): Pass relocation flag to
	darwin_rodata_section ().

From-SVN: r276623