RISC-V: Better support for long instructions (disassembler)

RISC-V: Better support for long instructions (disassembler)

Commit bb996692bd96 ("RISC-V/gas: allow generating up to 176-bit
instructions with .insn") tried to start supporting long instructions but
it was insufficient.

On the disassembler, correct ".byte" output was limited to the first 64-bits
of an instruction.  After that, zeroes are incorrectly printed.

Note that, it only happens on ".byte" output (instruction part) and not on
hexdump (data) part.  For example, before this commit, hexdump and ".byte"
produces different values:

Assembly:
  .insn 22, 0xfedcba98765432100123456789abcdef55aa33cc607f
objdump output example (before the fix):
  10:   607f 33cc 55aa cdef     .byte   0x7f, 0x60, 0xcc, 0x33, 0xaa, 0x55, 0xef, 0xcd, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
  18:   89ab 4567 0123 3210
  20:   7654 ba98 fedc

Note that, after 0xcd (after first 64-bits of the target instruction), all
".byte" values are incorrectly printed as zero while hexdump prints correct
instruction bits.

To resolve this, this commit adds "packet" argument to support dumping
instructions longer than 64-bits (to print correct instruction bits on
".byte").  This commit will be tested on the separate commit.

Assembly:
  .insn 22, 0xfedcba98765432100123456789abcdef55aa33cc607f
objdump output example (after the fix):
  10:   607f 33cc 55aa cdef     .byte   0x7f, 0x60, 0xcc, 0x33, 0xaa, 0x55, 0xef, 0xcd, 0xab, 0x89, 0x67, 0x45, 0x23, 0x01, 0x10, 0x32, 0x54, 0x76, 0x98, 0xba, 0xdc, 0xfe
  18:   89ab 4567 0123 3210
  20:   7654 ba98 fedc

opcodes/ChangeLog:

	* riscv-dis.c (riscv_disassemble_insn): Print unknown instruction
	using the new argument packet.
	(riscv_disassemble_data): Add unused argument packet.
	(print_insn_riscv): Pass packet to the disassemble function.