This patch moves all -march parsing logic into bfd, because we will use this
code in ELF attributes.

	bfd/
	* elfxx-riscv.h (RISCV_DONT_CARE_VERSION): New macro.
	(struct riscv_subset_t): New structure.
	(riscv_subset_t): New typedef.
	(riscv_subset_list_t): New structure.
	(riscv_release_subset_list): New prototype.
	(riscv_add_subset): Likewise.
	(riscv_lookup_subset): Likewise.
	(riscv_lookup_subset_version): Likewise.
	(riscv_release_subset_list): Likewise.
	* elfxx-riscv.c: Include safe-ctype.h.
	(riscv_parsing_subset_version): New function.
	(riscv_supported_std_ext): Likewise.
	(riscv_parse_std_ext): Likewise.
	(riscv_parse_sv_or_non_std_ext): Likewise.
	(riscv_parse_subset): Likewise.
	(riscv_add_subset): Likewise.
	(riscv_lookup_subset): Likewise.
	(riscv_lookup_subset_version): Likewise.
	(riscv_release_subset_list): Likewise.
	gas/
	* config/tc-riscv.c: Include elfxx-riscv.h.
	(struct riscv_subset): Removed.
	(riscv_subsets): Change type to riscv_subset_list_t.
	(riscv_subset_supports): Removed argument: xlen_required and move
	logic into libbfd.
	(riscv_multi_subset_supports): Removed argument: xlen_required.
	(riscv_clear_subsets): Removed.
	(riscv_add_subset): Ditto.
	(riscv_set_arch): Extract parsing logic into libbfd.
	(riscv_ip): Update argument for riscv_multi_subset_supports and
	riscv_subset_supports. Update riscv_subsets due to struct definition
	changed.
	(riscv_after_parse_args): Update riscv_subsets due to struct
	definition changed, update and argument for riscv_subset_supports.
	* testsuite/gas/riscv/empty.s: New.
	* testsuite/gas/riscv/march-fail-rv32ef.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ef.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32i.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32i.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32iam.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32iam.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ic.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ic.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32icx2p.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32icx2p.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32imc.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv32imc.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64I.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv64I.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64e.d: Likewise.
	* testsuite/gas/riscv/march-fail-rv64e.l: Likewise.
	* testsuite/gas/riscv/march-ok-g2.d: Likewise.
	* testsuite/gas/riscv/march-ok-g2p0.d: Likewise.
	* testsuite/gas/riscv/march-ok-i2p0.d: Likewise.
	* testsuite/gas/riscv/march-ok-nse-with-version.: Likewise.d
	* testsuite/gas/riscv/march-ok-s-with-version.d: Likewise.
	* testsuite/gas/riscv/march-ok-s.d: Likewise.
	* testsuite/gas/riscv/march-ok-sx.d: Likewise.
	* testsuite/gas/riscv/march-ok-two-nse.d: Likewise.
	* testsuite/gas/riscv/march-ok-g2_p1.d: Likewise.
	* testsuite/gas/riscv/march-ok-i2p0m2_a2f2.d: Likewise.
	include/
	* opcode/riscv.h (riscv_opcode): Change type of xlen_requirement to
	unsigned.
	opcodes/
	* riscv-opc.c: Change the type of xlen, because type of
	xlen_requirement changed.