ppc64le-rhel bot is changed to use unifiedtree builder instead as it has
a factory method with cmake and ninja.
I'm not exactly sure about nt-flags in the original clang builder, so
I would like to see how the buildbot handles this for now. The bot
will be kept on staging for now.
This may get same bug as ClangBuilder. Can you review https://reviews.llvm.org/D99685 or even integrate it into this, so that we will get compiler-rt changes in buildbot as well.