This prevents compilation failure we want to document a platform-specific
module. Every function is replaced by `loop {}` using the same construct
as `--unpretty everybody_loops`.

Note also a workaround to rust-lang#43636 is included: `const fn` will retain their
bodies, since the standard library has quite a number of them.

This attribute has two effects:

1. Items with this attribute and their children will have the "This is
   supported on **** only" message attached in the documentation.

2. The items' doc tests will be skipped if the configuration does not
   match.