"platform process list --arch" does not work unless combined with other options

Expected behaviour: To be able to `platform process list --arch aarch64` and get back all processes on the AArch64 Linux system the platform was running on.

Actual behaviour: I have to add another option for it to work, for example `platform process list --arch aarch64 --regex ".*"`

```
$ ./bin/lldb-server p --listen 127.0.0.1:1234
```
```
$ ~/build-llvm-aarch64/bin/lldb
(lldb) platform select remote-linux
Platform: remote-linux
Connected: no
(lldb) platform connect connect://127.0.0.1:1234
Platform: remote-linux
Triple: aarch64-unknown-linux-gnu
<...>
(lldb) platform process list --arch aarch64
error: invalid combination of options for the given command
(lldb) platform process list --arch aarch64 --contains ssh
error: no processes were found that contained "ssh" on the "remote-linux" platform
(lldb) platform process list --arch aarch64 --regex ".*"
15 matching processes were found on "remote-linux"
whose name matched the regular expression ".*"
PID PARENT USER TRIPLE NAME
====== ====== ========== ============================== ============================
1427 1 davspi01 aarch64-unknown-linux-gnu systemd
1511 1510 davspi01 aarch64-unknown-linux-gnu bash
```

Desired: either `--arch` only has a better error message, or it acts like `--arch `.