llvm: Adjust default supported targets
llvm: Adjust default supported targets

The default install for llvm should just be the common typical case, i.e.
support for local host and cpu architectures.  Enablingsupport for the wide
array of auxiliary architectures should be explicit rather than implicit.

LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack