RISC-V: Preserve arch version info during normalizing arch string

RISC-V: Preserve arch version info during normalizing arch string

- Arch version should preserved if user explicitly specified the version.
  e.g.
    After normalize, -march=rv32if3d should be -march=rv32i_f3p0d
    instead of-march=rv32ifd.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (riscv_subset_t): New field
	added.
	(riscv_subset_list::parsing_subset_version): Add parameter for
	indicate explicitly version, and handle explicitly version.
	(riscv_subset_list::handle_implied_ext): Ditto.
	(riscv_subset_list::add): Ditto.
	(riscv_subset_t::riscv_subset_t): Init new field.
	(riscv_subset_list::to_string): Always output version info if version
	explicitly specified.
	(riscv_subset_list::parsing_subset_version): Handle explicitly
	arch version.
	(riscv_subset_list::parse_std_ext): Ditto.
	(riscv_subset_list::parse_multiletter_ext): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/attribute-13.c: New.