condition: add CPUFeature

condition: add CPUFeature

Taking a stab at implementing #14479.

Add {Condition,Assert}CPUFeature to `systemd-analyze` & friends. Implement it
by executing the CPUID instruction. Add tables for common x86/i386
features.

Tested via unit tests + checked that commands such as:

```bash
systemd-analyze condition 'AssertCPUFeature = rdrand'
```

Succeed as expected and that commands such as

```bash
systemd-analyze condition 'AssertCPUFeature = foobar'
```

Fail as expected. Finally, I have amended the `systemd.unit` manual page
with the new condition and the list of all currently supported flags.