PR gold/11985

	PR gold/11985
	* layout.cc (Layout::create_initial_dynamic_sections): Don't crash
	if linker script discards key sections.
	(Layout::create_dynamic_symtab): Likewise.
	(Layout::assign_local_dynsym_offsets): Likewise.
	(Layout::sized_create_version_sections): Likewise.
	(Layout::create_interp): Likewise.
	(Layout::finish_dynamic_section): Likewise.
	(Layout::set_dynamic_symbol_size): Likewise.