Add cgroupv1 support to available_parallelism
Fixes#97549
My dev machine uses cgroup v2 so I was only able to test that code path. So the v1 code path is written only based on documentation. I could use some help testing that it works on a machine with cgroups v1:
```
$ x.py build --stage 1
# quota.rs
fn main() {
println!("{:?}", std::thread::available_parallelism());
}
# assuming stage1 is linked in rustup
$ rust +stage1 quota.rs
# spawn a new cgroup scope for the current user
$ sudo systemd-run -p CPUQuota="300%" --uid=$(id -u) -tdS
# should print Ok(3)
$ ./quota
```
If it doesn't work as expected an strace, the contents of `/proc/self/cgroups` and the structure of `/sys/fs/cgroups` would help.
bors
e55c53c57e