RISC-V: Drop riscv_ext_version_table in favor of riscv_ext_info_t data

RISC-V: Drop riscv_ext_version_table in favor of riscv_ext_info_t data

This commit drops the riscv_ext_version_table and instead uses the
riscv_ext_info_t data structure to provide the version information
for RISC-V extensions.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_ext_version_table):
	Remove.
	(standard_extensions_p): Use riscv_ext_info_t.
	(get_default_version): Use riscv_ext_info_t.
	(riscv_arch_help): Ditto.