RISC-V: Relax the -march string for accept any order

RISC-V: Relax the -march string for accept any order

-march was require canonical order before, however it's not easy for
most user when we have so many extension, so this patch is relax the
constraint, -march accept the ISA string in any order, it only has few
requirement:

1. Must start with rv[32|64][e|i|g].
2. Multi-letter and single letter extension must be separated by
   at least one underscore(`_`).

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::parse_single_std_ext): New parameter.
	(riscv_subset_list::parse_single_multiletter_ext): Ditto.
	(riscv_subset_list::parse_single_ext): Ditto.
	(riscv_subset_list::parse): Relax the order for the input of ISA
	string.
	* config/riscv/riscv-subset.h
	(riscv_subset_list::parse_single_std_ext): New parameter.
	(riscv_subset_list::parse_single_multiletter_ext): Ditto.
	(riscv_subset_list::parse_single_ext): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-33.c: New.
	* gcc.target/riscv/arch-34.c: New.