For the NeXT 64b ABI, IVAR refs are supposed to be indirected for
Mach-O PIC.  Identify them so that we can act as needed.

gcc/objc/ChangeLog:

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

	* objc-next-metadata-tags.h (OCTI_RT_META_IVAR_REF): New.
	(meta_ivar_ref): New.
	* objc-next-runtime-abi-02.c
	(next_runtime_abi_02_init_metadata_attributes): Create the
	IVAR ref metadata identifier.
	(ivar_offset_ref): Tag IVAR refs with specific metadata.

From-SVN: r276847