auto merge of #7040 : alexcrichton/rust/issue-6511-threadsafe-llvm, r=graydon

This is a reopening of #6570, and almost fixes #6511.

Note that this doesn't actually enable building a threadsafe LLVM, because that will require an LLVM rebuild which will be bundled with the upgrades in #6713.

What this does do, however, is removes all thread-unsafe usage of LLVM from the compiler.
Rewrite rusti