[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
1 file changed