preprocessor: Fix non-fn fn-like macro at EOF [PR97471]

preprocessor: Fix non-fn fn-like macro at EOF [PR97471]

We inject EOF tokens between macro argument lists, but had
confused/stale logic in the non-fn invocation.  Renamed the magic
'eof' token, as it's now only used for macro argument termination.
Always rewind the non-OPEN_PAREN token.

	libcpp/
	* internal.h (struct cpp_reader): Rename 'eof' field to 'endarg'.
	* init.c (cpp_create_reader): Adjust.
	* macro.c (collect_args): Use endarg for separator.  Always rewind
	in the not-fn case.
	gcc/testsuite/
	* c-c++-common/cpp/pr97471.c: New.