RISC-V: Allow instruction require more than one extension

RISC-V: Allow instruction require more than one extension

2018-08-29  Kito Cheng  <kito@andestech.com>

gas/
	* config/tc-riscv.c (riscv_subset_supports): New argument:
	xlen_required.
	(riscv_multi_subset_supports): New function, able to check more
	than one extension.
	(riscv_ip): Use riscv_multi_subset_supports instead of
	riscv_subset_supports.
	(riscv_set_arch): Update call-site for riscv_subset_supports.
	(riscv_after_parse_args): Likewise.

include/
	*opcode/riscv.h (MAX_SUBSET_NUM): New.
	(riscv_opcode): Add xlen_requirement field and change type of
	subset.

opcodes/
	* riscv-dis.c (riscv_disassemble_insn): Check XLEN by
	riscv_opcode.xlen_requirement.
	* riscv-opc.c (riscv_opcodes): Update for struct change.