[zorg][PowerPC] Don't checkout compiler-rt for runtime build

ClangBuilder will append all dependent projects to
DLLVM_ENABLE_PROJECTS, so we will get
'-DLLVM_ENABLE_PROJECTS=clang;compiler-rt;lld;llvm;clang-tools-extra'
-DLLVM_ENABLE_RUNTIMES=compiler-rt

This is breaking builds.
We shouldn't have compiler-rt in DLLVM_ENABLE_PROJECTS anymore.

Reviewed By: #powerpc, saghir, lei

Differential Revision: https://reviews.llvm.org/D90645