RISC-V: Minor improvements for dis-assembler.

* Extract all private_data initializations into riscv_init_disasm_info, which
called from print_insn_riscv rather than riscv_disassemble_insn.

* The disassemble_free_target seems like the right place to release all target
private_data, also including the internal data structures, like riscv_subsets.
Therefore, add a new function, disassemble_free_riscv, to release them for safe.

opcodes/
	* disassemble.c (disassemble_free_target): Called disassemble_free_riscv
	for riscv to release private_data and internal data structures.
	* disassemble.h: Added extern disassemble_free_riscv.
	* riscv-dis.c (riscv_init_disasm_info): New function, used to init
	riscv_private_data.
	(riscv_disassemble_insn): Moved riscv_private_data initializations
	into riscv_init_disasm_info.
	(print_insn_riscv): Called riscv_init_disasm_info to init
	riscv_private_data once time.
	(disassemble_free_riscv): New function, used to free the internal data
	structures, like riscv_subsets.