MMIX: Correct handling of C23 (...) functions, PR117618

This commit fixes a MMIX C23 (...)-handling bug; failing
gcc.dg/c23-stdarg-[46789].c execution tests.   But, this
isn't about a missing "|| arg.type != NULL_TREE" in the
PORT_setup_incoming_varargs function like most other
PR114175 port bugs exposed by the gcc.dg/c23-stdarg-6.c
.. -9.c tests; the MMIX port passes struct-return-values in
a register.  But, the bug is somewhat similar.

This bug seems like it was added already in
r13-3549-g4fe34cdcc80ac2, by incorrectly handling
TYPE_NO_NAMED_ARGS_STDARG_P-functions ((...)-functions);
counting them as having one parameter instead of none.  That
"+ 1" below is a kind-of hidden function_arg_advance call,
which shouldn't happen for (...)-functions.

	PR target/117618
	* config/mmix/mmix.cc (mmix_setup_incoming_varargs):
	Correct handling of C23 (...)-functions.