This commit prepares the `libc` crate to be included directly into the
standard library via crates.io. More details about this can be found on
rust-lang/rust#56092, but the main idea is that this crate now depends
on core/compiler-builtins explicitly (but off-by-default).

The main caveat here is that this activates `no_core` when building as
part of libstd, which means that it needs to explicitly have an `iter`
and `option` module for the expansion of `for` loops to work.

https://msdn.microsoft.com/en-us/library/windows/hardware/ff567981(v=vs.85).aspx

Matches corecrt_wtime.h from Win10 SDK

In preparation for rust-lang/rust#37545 this is adding the appropriate
directives to libc to get included.

Believe it or not, Windows actually defines a few POSIX
'E' error codes (eg EINVAL). By adding them to libc, the Windows
bindings for things like `read()` become useful.

This commit changes most uses of cfg_if as follows:
- fallthrough `else` usage is avoided for architecture or OS specific
  items
- a comment is added in the final `else` clause to signal intent someone
  modifying

It is safer to omit items than include ones for the wrong platform or
architecture.

It's tough to have PRs bounce or to have a back and forth with contributors
about minor style quibbles. Sometimes it ends up just being easier to fix style
after the fact, but let's add some automation to help this!

This commit adds a script to run on CI and locally to verify the style of this
repository. There's a few stylistic guidelines to ensure that definitions are
understandable across the jungle of modules. This consistency should help assist
readability for any future readers!

[ci skip]