RISC-V: Extend riscv_subset_list, preparatory for target attribute support

riscv_subset_list only accept a full arch string before, but we need to
parse single extension when supporting target attribute, also we may set
a riscv_subset_list directly rather than re-parsing the ISA string
again.

gcc/ChangeLog:

	* config/riscv/riscv-subset.h (riscv_subset_list::parse_single_std_ext):
	New.
	(riscv_subset_list::parse_single_multiletter_ext): Ditto.
	(riscv_subset_list::clone): Ditto.
	(riscv_subset_list::parse_single_ext): Ditto.
	(riscv_subset_list::set_loc): Ditto.
	(riscv_set_arch_by_subset_list): Ditto.
	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::parse_single_std_ext): New.
	(riscv_subset_list::parse_single_multiletter_ext): Ditto.
	(riscv_subset_list::clone): Ditto.
	(riscv_subset_list::parse_single_ext): Ditto.
	(riscv_subset_list::set_loc): Ditto.
	(riscv_set_arch_by_subset_list): Ditto.