RISC-V: Drop riscv_ext_flag_table in favor of riscv_ext_info_t data

RISC-V: Drop riscv_ext_flag_table in favor of riscv_ext_info_t data

Refactor extension flag handling by removing the old riscv_ext_flag_table and
sourcing all flag definitions directly from the flags field of the unified
riscv_ext_info_t structures generated from riscv-ext.def.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_extra_ext_flag_table_t):
	New.
	(riscv_ext_flag_table): Rename to ...
	(riscv_extra_ext_flag_table): this, and drop most of definitions
	that can obtained from the flags field of the riscv_ext_info_t
	structures.
	(apply_extra_extension_flags): Use riscv_ext_info_t.
	(riscv_ext_is_subset): Ditto.