gold/
	* options.h (General_options): Add -Trodata-segment option.
	* parameters.cc (Parameters::check_rodata_segment): New function.
	(Parameters::set_target_once): Call it.
	* parameters.h (Parameters): Declare it (private member function).
	* layout.cc (load_seg_unusable_for_headers): New function, broken
	out of Layout::relaxation_loop_body.  If TARGET->isolate_execinstr()
	then validate rodata segment rather than text segment.
	(relaxation_loop_body): Call that.
	(is_text_segment): New function.  Don't admit a non-executable
	segment if TARGET->isolate_execinstr().
	(set_segment_offsets): Call it.  Honor -Trodata-segment option.