mips: Fix C23 (...) functions returning large aggregates [PR114175]

mips: Fix C23 (...) functions returning large aggregates [PR114175]

We were assuming TYPE_NO_NAMED_ARGS_STDARG_P don't have any named
arguments and there is nothing to advance, but that is not the case
for (...) functions returning by hidden reference which have one such
artificial argument.  This is causing gcc.dg/c23-stdarg-{6,8,9}.c to
fail.

Fix the issue by checking if arg.type is NULL, as r14-9503 explains.

gcc/ChangeLog:

	PR target/114175
	* config/mips/mips.cc (mips_setup_incoming_varargs): Only skip
	mips_function_arg_advance for TYPE_NO_NAMED_ARGS_STDARG_P
	functions if arg.type is NULL.

(cherry picked from commit 6fc84f680d098f82c1c43435fdb206099f0df4df)