libcpp: Fix up handling of block comments in -fdirectives-only mode [PR103130]

Normal preprocessing, -fdirectives-only preprocessing before the Nathan's
rewrite, and all other compilers I've tried on godbolt treat even \*/
as end of a block comment, but the new -fdirectives-only handling doesn't.

2021-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/103130
	* lex.c (cpp_directive_only_process): Treat even \*/ as end of block
	comment.

	* c-c++-common/cpp/dir-only-9.c: New test.