RISC-V: Fix C ABI for flattened struct with 0-length bitfield.

RISC-V: Fix C ABI for flattened struct with 0-length bitfield.

	gcc/
	PR target/91229
	* config/riscv/riscv.c (riscv_flatten_aggregate_field): New arg
	ignore_zero_width_bit_field_p.  Skip zero size bitfields when true.
	Pass into recursive call.
	(riscv_flatten_aggregate_argument): New arg.  Pass to
	riscv_flatten_aggregate_field.
	(riscv_pass_aggregate_in_fpr_pair_p): New local warned.  Call
	riscv_flatten_aggregate_argument twice, with false and true as last
	arg.  Process result twice.  Compare results and warn if different.
	(riscv_pass_aggregate_in_fpr_and_gpr_p): Likewise.

	gcc/testsuite/
	* gcc.target/riscv/flattened-struct-abi-1.c: New test.
	* gcc.target/riscv/flattened-struct-abi-2.c: New test.

From-SVN: r274215