auto merge of #7113 : alexcrichton/rust/banned-warnings, r=cmr

Reopening of #7031, Closes #6963

I imagine though that this will bounce in bors once or twice... Because attributes can't be cfg(stage0)'d off, there's temporarily a lot of new stage0/stage1+ code.

diff --cc src/libstd/run.rs
Simple merge

diff --cc src/libstd/task/spawn.rs
Simple merge

diff --cc src/libstd/unstable/dynamic_lib.rs
Simple merge