Using cargo clippy with -Z build-std produces warnings inside of libcore

If you attempt to use `cargo clippy` with the new cargo support for building the standard libraries (obsoleting cargo-xbuild and xargo), clippy will emit tons of warnings inside of libcore.

Usage: `cargo clippy -Z build-std=core,alloc --target=<anything>`

You can see an example of this failing here: https://travis-ci.com/fishinabarrel/linux-kernel-module-rust/builds/128613174