When building clang with e.g. LLVM_ENABLE_RUNTIMES=libcxx;libunwind,
those runtimes end up in the stdlib search directory, and when
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is set, that ends up in a
target-specific subdirectory. The stdlib search does handle the
situation, but when the target in question is Android, the same issues
as those that required fallbacks for runtimes search apply.
Traditionally, those libraries are shipped as part of the Android NDK,
but when one builds their own clang for Android, they may want to use
the runtimes from the same version rather than the ones from the NDK.