The comment in lib/systems/default.nix for uname.processor indicates that it
should match `uname -p`. I tried that command and found that it reports
`unknown` on all of these machines:
- `x86_64-linux`
- `aarch64-linux`
- `mips64el-linux`
- `powerpc64le-linux`
The command `uname -m` reports the expected value on all of the above.
I think the comment is wrong. So I fixed it.