RISC-V: Support --target-help for -mcpu/-mtune

RISC-V: Support --target-help for -mcpu/-mtune

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_tunes): New.
	(riscv_get_valid_option_values): New.
	(TARGET_GET_VALID_OPTION_VALUES): New.
	* config/riscv/riscv-cores.def (RISCV_TUNE): New, define options
	for tune here.
	(RISCV_CORE): Fix comment.
	* config/riscv/riscv.cc (riscv_tune_info_table): Move definition to
	riscv-cores.def.