RISC-V: Suppress warning

RISC-V: Suppress warning

../../gcc/config/riscv/riscv.cc: In function 'void riscv_init_cumulative_args(CUMULATIVE_ARGS*, tree, rtx, tree, int)':
../../gcc/config/riscv/riscv.cc:4879:34: error: unused parameter 'fndecl' [-Werror=unused-parameter]
4879 |                             tree fndecl,
      |                             ~~~~~^~~~~~
../../gcc/config/riscv/riscv.cc: In function 'bool riscv_vector_mode_supported_any_target_p(machine_mode)':
../../gcc/config/riscv/riscv.cc:10537:56: error: unused parameter 'mode' [-Werror=unused-parameter]
10537 | riscv_vector_mode_supported_any_target_p (machine_mode mode)
      |                                           ~~~~~~~~~~~~~^~~~
cc1plus: all warnings being treated as errors
make[3]: *** [Makefile:2559: riscv.o] Error 1

Suppress these warnings.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_init_cumulative_args): Suppress warning.
	(riscv_vector_mode_supported_any_target_p): Ditto.