Fix styling

Re-run CI

Co-Authored-By: gnzlbg <gnzlbg@users.noreply.github.com>

Add a new feature to enable this, since `const extern fn`
support is unstable

Closes #981 .

This PR fixes the build on all platforms and all Rust version down to the
minimum Rust version supported by libc: Rust 1.13.0.

The `build.rs` is extended with logic to detect the newer Rust features used by
`libc` since Rust 1.13.0:

* Rust 1.19.0: `untagged_unions`. APIs using untagged unions are gated on
  `cfg(libc_unions)` and not available on older Rust versions.

* Rust 1.25.0: `repr(align)`. Because `repr(align)` cannot be parsed by older
  Rust versions, all uses of `repr(align)` are split into `align.rs` and
  `no_align.rs` modules, which are gated on the `cfg(libc_align)` at the top
  level. These modules sometimes contain macros that are expanded at the top
  level to avoid privacy issues (`pub(crate)` is not available in older Rust
  versions). Closes #1242 .

* Rust : `const` `mem::size_of`. These uses are worked around with hardcoded
  constants on older Rust versions.

Also, `repr(packed)` structs cannot automatically `derive()` some traits like
`Debug`. These have been moved into `s_no_extra_traits!` and the lint of missing
`Debug` implementations on public items is silenced for these. We can manually
implement the `extra_traits` for these in a follow up PR. This is tracked
in #1243. Also, `extra_traits` does not enable `align` manually anymore.

Since `f64::to_bits` is not available in older Rust versions, its usage
has been replaced with a `transmute` to an `u64` which is what that method
does under the hood.

Closes #1232 .

The libc crate is used as a dependency of the Rust compiler. Its build
system passes `--cfg dox` to all crates when generating their
documentation. libc's documentation is generated when the build system
is asked to generate the compiler documentation because `cargo doc`
automatically documents all dependencies.

When the dox configuration option is enabled, libc disables its
dependency on the core crate and provides the necessary definitions
itself. The dox configuration option is meant for generating
documentation for a multitude of targets even if the core crate for that
target is not installed. However, when documenting the compiler, it's
not necessary to do that; we can just use core or std as usual.

This change is motivated by the changes made to the compiler in
rust-lang/rust#48171. With these changes, it's necessary to provide
implementations of the Clone and Copy traits for some primitive types in
the library that defines these traits (previously, these implementations
were provided by the compiler). Normally, these traits (and thus the
implementations) are provided by core, so any crate that uses
`#![no_core]` must now provide its own copy of the implementations.

Because libc doesn't provide its own copy of the implementations yet,
and because the compiler's build system passes `--cfg dox` to libc,
generating the documentation for the compiler fails when generating
documentation for libc. By renaming the configuration option, libc will
use core or std and will thus have the necessary definitions for the
documentation to be generated successfully.

We now create an additional binary `linux_fcntl` for testing this
since there are header conflicts when including all necessary headers.
This binary is run on all platforms even though it's empty on all non-
Android/non-Linux platforms.

Testing has been switched from a custom binary to using a runner-less
test (or pair of tests). This means that for local development a simple
`cd libc-test && cargo test` will run all the tests. CI has also been
updated here to reflect that.

Since #[cfg] attributes are short-circuting, cfg_if! should not change
the order in which the predicates are listed. This enables the use of
unstable cfg attributes in cfg_if!, which is useful when building libstd.

[ci skip]