[GOLD] reporting local symbol names

[GOLD] reporting local symbol names

get_symbol_name currently returns "" for the usual STT_SECTION symbols
generated by gas.  That's not very helpful, return the section name.
Demangle local symbols too, fixing an inconsistency in
issue_discarded_error where global symbols are demangled.

	* object.cc (Sized_relobj_file::get_symbol_name): Return a
	std::string.  Return section name for STT_SECTION symbols with
	zero st_name.  Sanity check st_name, and don't run off the end
	of an improperly terminated .strtab.  Demangle sym names.
	* object.h (Sized_relobj_file::get_symbol_name): Update decl.
	* target-reloc.h (issue_discarded_error): Adjust.
	* powerpc.cc (Target_powerpc::Relocate::relocate): Report reloc
	type and symbol for relocation overflows.