Disable non-functional ppc64 support for ldc packages
Sadly, ldc ppc64 support has detoriated so much that it no longer works
with any current llvm version. I'll keep an eye on things and re-enable
it once it's fixed, but right now it's just broken and upstream is
suggesting to disable the support for now.
https://github.com/ldc-developers/ldc/issues/2356