auto merge of #8534 : huonw/rust/tls-key-macro, r=alexcrichton

This allows the internal implementation details of the TLS keys to be
changed without requiring the update of all the users. (Or, applying
changes that *have* to be applied for the keys to work correctly, e.g.
forcing LLVM to not merge these constants.)

diff --cc src/libsyntax/ext/expand.rs
Simple merge