opcodes: fix Werror=format build breaker in opcodes/riscv-dis.c

opcodes: fix Werror=format build breaker in opcodes/riscv-dis.c

I build gdb on arm-linux and ran into:
...
  CC       riscv-dis.lo
opcodes/riscv-dis.c: In function ‘print_insn_args’:
opcodes/riscv-dis.c:743:29: error: format ‘%lu’ expects argument of type \
  ‘long unsigned int’, but argument 4 has type ‘insn_t’ \
  {aka ‘long long unsigned int’} [-Werror=format=]
  743 |                          "%lu", EXTRACT_ZCMT_INDEX (l));
      |                           ~~^
      |                             |
      |                             long unsigned int
      |                           %llu
...

Fix this by printing the insn_t value, which is a uint64_t, using PRIu64.

Tested by finishing the build.