Auto merge of #1999 - Jakob-Naucke:fix-statfs-magic-s390x, r=JohnTitor
Auto merge of #1999 - Jakob-Naucke:fix-statfs-magic-s390x, r=JohnTitor

Declare statfs MAGIC constants as c_uint on s390x

Hi, I work in container development on Linux on Z at IBM.
On s390x (IBM Z) in GNU/Linux, a statfs `f_type` is 4 bytes wide, contrary to 8 bytes on most architectures.
This is already implemented in libc:
```sh
$ grep -r f_type src/unix/linux_like/linux/gnu/b64 | uniq
src/unix/linux_like/linux/gnu/b64/sparc64/mod.rs:        pub f_type: ::__fsword_t,
src/unix/linux_like/linux/gnu/b64/aarch64/mod.rs:        pub f_type: ::__fsword_t,
src/unix/linux_like/linux/gnu/b64/powerpc64/mod.rs:        pub f_type: ::__fsword_t,
src/unix/linux_like/linux/gnu/b64/riscv64/mod.rs:        pub f_type: ::c_long,
src/unix/linux_like/linux/gnu/b64/mips64/mod.rs:        pub f_type: ::c_long,
src/unix/linux_like/linux/gnu/b64/s390x.rs:        pub f_type: ::c_uint,             # s390x is uint
src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs:        pub f_type: ::__fsword_t,
```
However, the `*_MAGIC` constants (such as `EXT4_SUPER_MAGIC`) in `src/unix/linux_like/linux/gnu/mod.rs` are defined as `c_long`, when they should be `c_uint` on s390x.
This ends up biting me [here](https://github.com/kata-containers/cgroups-rs/blob/master/src/hierarchies.rs#L231).
Thus, I suggest the attached change to only define these constants for architectures other than s390x and instead define them as uint for s390x.
This is safe since none of the constants are any wider than 32bit.
Please let me know if you think this could be done more elegantly.

rust-lang/libc,rust-lang/libc,rust-lang/libc