Initialize LLVM only once

This also appears to fix a race in LLVM that was causing a deadlock on the bots
during the doc-test tests (where we use rustc in parallel).
auto merge of #11170 : alexcrichton/rust/fix-llvm-race, r=pcwalton

This also appears to fix a race in LLVM that was causing a deadlock on the bots
during the doc-test tests (where we use rustc in parallel).