LLVM shouldn't be build with `--disable-threads`

[alexcrichton]

After #6511 and #7040, rustc is ready to use LLVM completely threadsafely.

All that's missing is to stop passing the --disable-threads and --disable-pthreads flags to LLVM's configure script. Once this is done, the lock-enabling code in trans should be uncommented as well.

This is a simple change, but requires the bots to rebuild LLVM (which has caused problems in the past). @brson has some other LLVM patches to integrate, and this would fit nicely with those.

[catamorphism]

#6511 and #7040 are closed now, so this should be ready to be closed as well. I'm not sure what the problems are with the bots rebuilding LLVM (@brson?) so I'm a bit hesitant to try it myself.

I'd like this to be fixed so I can re-enable the rustpkg tests, though.

[alexcrichton]

@catamorphism, I just opened a pull request (#7393) to actually stop passing this option to configuring LLVM.

The problem is that this doesn't trigger an LLVM rebuild, so once the change has gone through the bots need to have their llvm checkout cleaned before landing the further improvements (uncommenting that block of code to enable a threadsafe rustc)

[brson]

We can try running the bots with clean-llvm after that patch lands, but I don't know if it works. Without graydon I think we can at least manually clean the windows, mac, and bsd bots.