various xcoff rs6000 linker issues

This patch started out as an exercise in sanity checking r_symndx
before indexing symbol hashes, but grew as I noticed other issues.

Merges xcoff_ppc_relocate_section and xcoff64_ppc_relocate_section.
All of the xcoff_reloc_function now return a status, with the idea
that error reporting can be done in one place.

	* libxcoff.h (xcoff_reloc_function): Return bfd_reloc_status_type
	rather than bool.
	(_bfd_xcoff_relocate_section): Declare.
	* coff-rs6000.c (xcoff_reloc_type_noop, xcoff_reloc_type_fail),
	(xcoff_reloc_type_pos, xcoff_reloc_type_neg, xcoff_reloc_type_rel),
	(xcoff_reloc_type_toc, xcoff_reloc_type_ba, xcoff_reloc_type_br),
	(xcoff_reloc_type_crel, xcoff_reloc_type_tls): Return
	bfd_reloc_status_type rather than bool.  Don't emit standard
	error message and don't set bfd error.  Replace r_symndx
	non-negative check with check against obj_raw_syment_count.
	(_bfd_xcoff_relocate_section): Extracted from
	xcoff_ppc_relocate_section and xcoff64_ppc_relocate_section.
	Move error reporting to end of loop.  Report error for
	r_symndx out of range.  Adjust for reloc howto special
	function change.  Properly check that reloc offset is in range.
	Prevent possible segfault in _bfd_coff_internal_syment_name.
	* coff64-rs6000.c: Similarly.