[zorg] Use boolean flag to determine if building a unified standalone runtimes
For AIX libcxx build, we use ../llvm/runtimes as build source and with
-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" option, since we are using
the new runtimes build laided out by the "A vision for building the runtimes"
RFC and D93408.
Differential Revision: https://reviews.llvm.org/D97326