[libcxx] Add a CI configuration for standalone building in llvm-project/runtimes Generate the llvm-lit script in runtimes/CmakeFiles.txt unless invoked from llvm/runtimes. Differential Revision: https://reviews.llvm.org/D109593 GitOrigin-RevId: 565d45541f860d72b2e2f12001104edd78f5b0c3