Remove powerpc.cc copy of use_plt_offset

Remove powerpc.cc copy of use_plt_offset

This adds an extra flag for needs_dynamic_reloc() in order to remove
the copy of this function and use_plt_offset() in powerpc.cc, and
tweaks the powerpc get_reference_flags() to return the flag as
appropriate.  ELFv2 does not want ELFv1 behaviour here.

	* symtab.h (Symbol::Reference_flags): Add FUNC_DESC_ABI.
	(Symbol::needs_dynamic_reloc): Test new flag.
	* powerpc.cc (needs_dynamic_reloc, use_plt_offset): Delete.
	(Target_powerpc::Scan::get_reference_flags): Add target param.
	Return FUNC_DESC_ABI for 64-bit ELFv1.
	(Target_powerpc::Branch_info::make_stub): Adjust get_reference_flags
	call.
	(Target_powerpc::Scan::global): Use Symbol::needs_dynamic_reloc.
	(Target_powerpc::Relocate::relocate): Use Symbol::use_plt_offset.