* target-reloc.h (scan_relocs): Call scan.local for relocs

	* target-reloc.h (scan_relocs): Call scan.local for relocs
	against symbols in discarded sections.  Pass is_discarded
	param.
	* arm.cc, * i386.cc, * sparc.cc, * x86_64.cc (Target_*::Scan::local):
	Add is_discarded param.
	* powerpc (Target_powerpc::Scan::local): Likewise.  Use
	is_discarded to flag opd entry as discarded.  Don't emit dyn
	relocs on such entries.
	(Target_powerpc::Scan::global): Similarly detect and handle
	such opd entries.
	(Powerpc_relobj): Replace opd_ent_shndx_ and opd_ent_off_ with
	opd_ent_.  Update all uses.
	(Powerpc_relobj::get_opd_discard, set_opd_discard): New functions.
	(Target_powerpc::relocate_section): Zero out discarded opd
	entry relocs.