Rollup merge of #25140 - kevinmehall:mips, r=steveklabnik

Rollup merge of #25140 - kevinmehall:mips, r=steveklabnik

Building with `--target=mipsel-unknown-linux-gnu` currently results in the following errors, fixed by this PR:

```
rustc: x86_64-unknown-linux-gnu/stage2/lib/rustlib/mipsel-unknown-linux-gnu/lib/libstd
/vol/rust/src/libstd/os/linux/raw.rs:76:21: 76:28 error: use of undeclared type name `c_ulong`
/vol/rust/src/libstd/os/linux/raw.rs:76         pub st_dev: c_ulong,
                                                            ^~~~~~~
/vol/rust/src/libstd/os/linux/raw.rs:83:22: 83:29 error: use of undeclared type name `c_ulong`
/vol/rust/src/libstd/os/linux/raw.rs:83         pub st_rdev: c_ulong,
                                                             ^~~~~~~
/vol/rust/src/libstd/sys/common/net2.rs:210:52: 210:70 error: unresolved name `libc::TCP_KEEPIDLE`
/vol/rust/src/libstd/sys/common/net2.rs:210         setsockopt(&self.inner, libc::IPPROTO_TCP, libc::TCP_KEEPIDLE,
```