opcodes/riscv: add disassembler options support to libopcodes

opcodes/riscv: add disassembler options support to libopcodes

In preparation for the next commit, which will add GDB support for
RISC-V disassembler options, this commit restructures how the
disassembler options are managed within libopcodes.

The implementation provided here is based on this mailing list patch
which was never committed:

  https://sourceware.org/pipermail/binutils/2021-January/114944.html

which in turn took inspiration from the MIPS implementation of the
same feature.

The biggest changes from the original mailing list post are:

  1. The GDB changes have been split into a separate patch, and

  2. The `riscv_option_args_privspec` variable, which held the valid
  priv-spec values is now gone, instead we use the `riscv_priv_specs`
  array from bfd/cpu-riscv.c instead.

Co-authored-by: Simon Cook <simon.cook@embecosm.com>
include/ChangeLog:

	* dis-asm.h (disassembler_options_riscv): Declare.

opcodes/ChangeLog:

	* riscv-dis.c (enum riscv_option_arg_t): New enum typedef.
	(riscv_options): New static global.
	(disassembler_options_riscv): New function.
	(print_riscv_disassembler_options): Rewrite to use
	disassembler_options_riscv.