Support for specifying the code model

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.
mk: Fix LD_LIBRARY_PATH_ENV_NAMES in cross build
obsolete.rs: fix typo in message
rustc.1: fix typo

Uses the same wording as ``config.rs`` now.
complement-lang-faq.md: fix typos
Main example for bigint usage.

Also use `///` for documentation instead of `/**`. End comments with
`.`.

Show imports for bigint example.

They might be useful.
Improve docs on JSON.

- add one simple example of using `ToJson` trait
- make example code more readable
auto merge of #15698 : Zoxc/rust/code-model, r=alexcrichton

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.

Testing for this would be fragile and very architecture specific and is better left to LLVM.
auto merge of #15675 : errordeveloper/rust/json_docs, r=steveklabnik

- add one simple example of using `ToJson` trait
- make example code more readable
auto merge of #15705 : klutzy/rust/cross-ld-library, r=alexcrichton
auto merge of #15706 : phi-gamma/rust/master, r=huonw

I kept changes to each file in a separate commit. Please let me know if you prefer them
squashed!
auto merge of #15716 : treeman/rust/bigint-doc, r=alexcrichton

Also use `///` for documentation instead of `/**`. End comments with `.`.