-cpu <whatever>,help should print the options available for that CPU type

The '-device' option allows the user to list the options supported by a given device, like this:

$ qemu-system-x86_64 -device tulip,help
tulip options:
  acpi-index=<uint32>    -  (default: 0)
  addr=<int32>           - Slot and optional function number, example: 06.0 or 06 (default: -1)
  bootindex=<int32>
  failover_pair_id=<str>
  mac=<str>              - Ethernet 6-byte MAC Address, example: 52:54:00:12:34:56
  multifunction=<bool>   - on/off (default: false)
  netdev=<str>           - ID of a netdev to use as a backend
  rombar=<uint32>        -  (default: 1)
  romfile=<str>
  romsize=<uint32>       -  (default: 4294967295)
  x-pcie-extcap-init=<bool> - on/off (default: true)
  x-pcie-lnksta-dllla=<bool> - on/off (default: true)

However, if you try the same for -cpu it doesn't work:

$ qemu-system-x86_64 -cpu athlon,help
qemu-system-x86_64: can't apply global athlon-x86_64-cpu.help=on: Property 'athlon-x86_64-cpu.help' not found

It would be nice if we supported 'help' here in a similar way that we do for the -device option.

(Noted in this stackoverflow query: https://stackoverflow.com/questions/75383332/how-to-check-the-configuration-keys-available-for-a-qemu-cpu/75387635#75387635)