[gold] Implement -z stack-size option

[gold] Implement -z stack-size option

gold/
	* options.h (General_options): Grok -z stack-size.
	* output.h (Output_segment::set_size): New method.
	* layout.cc (Layout::create_executable_stack_info): Renamed to ...
	(Layout::create_stack_segment): ... this.  Always create the
	segment if -z stack-size was used.
	(Layout::set_segment_offsets): Don't call ->set_offset on the
	PT_GNU_STACK segment.