Rollup merge of #43342 - ranweiler:no-std-exe-docs, r=alexcrichton

Document use of `compiler_builtins` with `no_std` binaries

See discussion in #43264.

The docs for the `compiler_builtins_lib` feature were removed in
PR #42899. But, though the `compiler_builtins` library has been
migrated out-of-tree, the language feature remains, and is needed to
use the stand-alone crate. So, we reintroduce the docs for the
feature, and add a reference to them when describing how to create a
`no_std` executable.

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