RISC-V: Fix C23 (...) functions returning large aggregates [PR114175]

RISC-V: Fix C23 (...) functions returning large aggregates [PR114175]

We assume that 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 causes gcc.dg/c23-stdarg-[68].c to fail

Fix the issue by checking if arg.type is NULL as r14-9503-g218d1749612
explains

Tested on linux rv64gcv.

gcc/ChangeLog:

	PR target/114175
	* config/riscv/riscv.cc (riscv_setup_incoming_varargs): Only skip
	riscv_funciton_arg_advance for TYPE_NO_NAMED_ARGS_STDARG_P functions
	if arg.type is NULL