RISC-V: Move certain arrays to riscv-opc.c

This is a part of small tidying (declare tables in riscv-opc.c).

include/ChangeLog:

	* opcode/riscv.h (riscv_rm, riscv_pred_succ): Move declarations to
	opcodes/riscv-opc.c.  New non-static definitions.

opcodes/ChangeLog:

	* riscv-opc.c (riscv_rm, riscv_pred_succ): Move from
	include/opcode/riscv.h.  Add description.