RISC-V: Gate opcode tables by enum rather than string.

RISC-V: Gate opcode tables by enum rather than string.

Generalize opcode arch dependencies so that we can support the
overlapping B extension Zb* subsets.

	2019-09-17  Maxim Blinov  <maxim.blinov@embecosm.com>
	gas/
	* config/tc-riscv.c (riscv_multi_subset_supports): Handle
	insn_class enum rather than subset char string.
	(riscv_ip): Update call to riscv_multi_subset_supports.
	include/
	* opcode/riscv.h (riscv_insn_class): New enum.
	* opcode/riscv.h (struct riscv_opcode): Change
	subset field to insn_class field.
	opcodes/
	* riscv-opc.c (riscv_opcodes): Change subset field
	to insn_class field for all instructions.
	(riscv_insn_types): Likewise.