Consider enabling LLVM_ENABLE_THREADS

[vgvassilev]

As per https://github.com/root-project/root/blob/a05d4beded1481a86a65e450826352eb528b03e7/interpreter/CMakeLists.txt#L51 we explicitly disable the llvm threading support. That leaves us on a quite untested code especially with the ORCV2 infrastructure which makes use of asynchronous compilation.