llvm not built correctly on PowerPC

Superbuild needs to be updated to correctly target PowerPC. Specifically, in file superbuild projects/unix/llvm.cmake, line 28,

-DLLVM_TARGETS_TO_BUILD:STRING=X86 # FIXME: When using this on PowerPC,

This needs to be fixed for all PowerPC clusters.

NOTE: Within the next few months, we will also need to be able to build ARM based versions of ParaView.