Large parts of (e.g.) `sched.h` not accessible without `_GNU_SOURCE`

I'm experimenting with using Swift for some low(ish)-level Linux system programming, and I'd like to invoke the [unshare(2)][unshare(2)] system call to create a new namespace.

`unshare` is normally exposed in C by `sched.h` (ultimately `bits/sched.h`). CSystem on Linux [includes `sched.h`][CSystemLinux.h]; however, as it doesn't define `_GNU_SOURCE`, most of `bits/sched.h` is not exposed, including the namespace (containerisation) functions `clone`, `unshare`, and `setns` (and their associated `CLONE_*` flags).

I understand the primary purpose of CSystem is to expose functionality that is then wrapped by System; and maybe one day System on Linux will include a wrapper for Linux namespaces: they're a compelling feature for the platform. Unfortunately, I'm not yet experienced enough with Swift to volunteer to write one.

In any case, would it be possible to `#define _GNU_SOURCE` in CSystem on Linux? What problems (if any) could this cause?

Other desirable system interfaces, often including safer alternatives to standard interfaces, are also gated behind it, e.g. `get_current_dir_name`, which was suggested for use in System in https://github.com/apple/swift-system/pull/71#issuecomment-968160476.

[unshare(2)]: https://manpages.debian.org/bookworm/manpages-dev/unshare.2.en.html
[CSystemLinux.h]: https://github.com/apple/swift-system/blob/main/Sources/CSystem/include/CSystemLinux.h

Start with Sources/CSystem/include/CSystemLinux.h and the sched.h inclusion described in the issue. Check how defining _GNU_SOURCE affects the exposed namespace APIs, CLONE_* flags, and other gated interfaces, then verify that CSystem still builds cleanly on Linux without causing compatibility problems.

Written by the indexing model from the issue text.