auto merge of #10312 : thestinger/rust/thread_local, r=alexcritchton

This provides a building block for fast thread-local storage. It does
not change the safety semantics of `static mut`.

Closes #10310

diff --cc doc/rust.md
Simple merge

diff --cc src/librustc/lib/llvm.rs
Simple merge

diff --cc src/librustc/middle/lint.rs
Simple merge

diff --cc src/librustc/middle/trans/base.rs
Simple merge

diff --cc src/libstd/rt/local_ptr.rs
Simple merge

diff --cc src/libstd/rt/mod.rs
Simple merge