RISC-V: Extract part parsing base ISA logic into a standalone function [NFC]

RISC-V: Extract part parsing base ISA logic into a standalone function [NFC]

Minor refactor, preparation for further change.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::parse_base_ext): New.
	(riscv_subset_list::parse): Extract part of logic into
	riscv_subset_list::parse_base_ext.
	* config/riscv/riscv-subset.h (riscv_subset_list::parse_base_ext):
	New.