mask `compiler_builtins` docs
mask `compiler_builtins` docs

Since `compiler_builtins` is being injected automatically, its docs
aren't masked. This commit masks the crate's docs if it's brought in as
an extern crate.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust