Add 'default' to -foffload=; document that flag [PR67300]

Add 'default' to -foffload=; document that flag [PR67300]

As -foffload={options,targets,targets=options} is very convoluted,
it has been split into -foffload=targets (supporting the old syntax
for backward compatibilty) and -foffload-options={options,target=options}.

Only the new syntax is documented.

Additionally, -foffload=default is supported, which can reset the
devices after -foffload=disable / -foffload=targets to the default,
if needed.

gcc/ChangeLog:

	PR other/67300
	* common.opt (-foffload=): Update description.
	(-foffload-options=): New.
	* doc/invoke.texi (C Language Options): Document
	-foffload and -foffload-options.
	* gcc.c (check_offload_target_name): New, split off from
	handle_foffload_option.
	(check_foffload_target_names): New.
	(handle_foffload_option): Handle -foffload=default.
	(driver_handle_option): Update for -foffload-options.
	* lto-opts.c (lto_write_options): Use -foffload-options
	instead of -foffload.
	* lto-wrapper.c (merge_and_complain, append_offload_options):
	Likewise.
	* opts.c (common_handle_option): Likewise.

libgomp/ChangeLog:

	PR other/67300
	* testsuite/libgomp.c-c++-common/reduction-16.c: Replace
	-foffload=nvptx-none= by -foffload-options=nvptx-none= to
	avoid disabling other offload targets.
	* testsuite/libgomp.c-c++-common/reduction-5.c: Likewise.
	* testsuite/libgomp.c-c++-common/reduction-6.c: Likewise.
	* testsuite/libgomp.c/target-44.c: Likewise.