Make flow of option processing more readily visible

Make flow of option processing more readily visible

This moves calls to various option processing stages to one place,
toplev::main.

2021-10-05  Richard Biener  <rguenther@suse.de>

	* toplev.c (no_backend): Remove global var.
	(process_options): Pass in no_backend, move post_options
	langhook call to toplev::main.
	(do_compile): Pass in no_backend, move process_options call
	to toplev::main.
	(toplev::run_self_tests): Check no_backend at the caller.
	(toplev::main): Call post_options and process_options
	split out from do_compile, do self-tests only if
	no_backend is initialized.