RISC-V: Handle implied extension in canonical ordering.

RISC-V: Handle implied extension in canonical ordering.

 - ISA spec has specify the order between multi-letter extensions, implied
   extension also need to follow store in canonical ordering, so
   most easy way is we keep that in-order during insertion.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (single_letter_subset_rank): New.
	(multi_letter_subset_rank): Ditto.
	(subset_cmp): Ditto.
	(riscv_subset_list::add): Insert subext in canonical ordering.
	(riscv_subset_list::parse_std_ext): Move handle_implied_ext to ...
	(riscv_subset_list::parse): ... here.