Because we cannot yet bring a more recent musl to the mips and mipsel
architectures, we disable support for these constant until a cascading
update with rust-lang/rust has been done.

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 .

These are defined in the fcntl.h glibc header on s390x systems
on Linux but missing in the libc crate, so add them as they are
required for the file locking API in rustc.

`pthread_mutex_t` varies across architectures, in several ways:
- endianness alters the ordering of bytes, since the contents of the
  struct are larger than 8-bit.
- its length varies.
- the location of the mutex kind (`PTHREAD_MUTEX_RECURSIVE`,
  `PTHREAD_MUTEX_ERRORCHECK` or `PTHREAD_MUTEX_ADAPTIVE_NP`) varies
  between 32-bit and 64-bit: On 32-bit architectures, it is preceded by
  three int/unsigned int, while on 64-bit architectures, it is preceded
  by four of them.

These initializers are only available from <pthread.h> when _GNU_SOURCE
is defined.

Relax the cfg_if check in ci/style.rs to allow #[cfg(target_endian)]
tests.

These aren't exposed on non-MIPS musl targets, but since they're
part of a kernel API, they're still applicable, so we just don't
test them there but expose them anyways.

Note that termios2 doesn't exist on powerpc(64), termios
is used instead.

* Add syscall tables to most remaining arches in `src/unix/notbsd`
* Add aarch64/arm musl to CI
* Update dependencies

Motivation: there is no glibc wrapper for this syscall

Also move them to the common Linux module, as they are not platform-specific.
See include/uapi/linux/falloc.h.

These are all that remain for nix to compile on s390x

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650