std: Allow creating ExitStatus from raw values

std: Allow creating ExitStatus from raw values

Sometimes a process may be waited on externally from the standard library, in
which case it can be useful to create a raw `ExitStatus` structure to return.
This commit extends the existing Unix `ExitStatusExt` extension trait and adds a
new Windows-specific `ExitStatusExt` extension trait to do this. The methods are
currently called `ExitStatus::from_raw`.

cc #32713