Enable AVX with LLVM trunk.
It seems that with LLVM trunk, an empty string as the MCPU parameter
to EngineBuilder::selectTarget no longer selects the host architecture.

Also remove superfluous logic in the use of EngineBuilder and method
selectTarget.  There were .setMCPU and .setMAttrs calls that were
pointless, because those calls set values that are not used later,
because later we pass the desired values explicitly to .setTarget.