[gdb] Enable some more print_one_insn selftests

In print_one_insn_test we have this cluster of skipped tests:
...
    case bfd_arch_ia64:
    case bfd_arch_mep:
    case bfd_arch_mips:
    case bfd_arch_tic6x:
    case bfd_arch_xtensa:
      return;
...

Enable some of these, and document in more detail why they're enabled or
skipped.

Likewise, document bfd_arch_or1k because it's an odd case.

Tested on x86_64-linux.