RISC-V: Refine riscv_parse_arch_string

RISC-V: Refine riscv_parse_arch_string

 - Generalize logic for translating arch to internal flags, this patch
   is infrastructure for supporing sub-extension parsing.

gcc/ChangeLog

	* common/config/riscv/riscv-common.c (opt_var_ref_t): New.
	(riscv_ext_flag_table_t): New.
	(riscv_ext_flag_table): New.
	(riscv_parse_arch_string): Pass gcc_options* instead of
	&opts->x_target_flags only, and using riscv_arch_option_table to
	setup flags.
	(riscv_handle_option): Update argument for riscv_parse_arch_string.
	(riscv_expand_arch): Ditto.
	(riscv_expand_arch_from_cpu): Ditto.