building with explicit settings of MAKE_INSTALL_FULL_LIBDIR and MAKE_INSTALL_LIBDIR has no effect

The Debian and Ubuntu packages are installed into a subdir /usr/lib/llvm-N (CMAKE_INSTALL_PREFIX). There are requests to make the binary packages Multi-Arch: same compatible. That means, you can install the same package for the default architecture, and for other architectures (e.g. amd64 and arm64). In the current install, libraries (*.so.*, *.a) are installed into PREFIX/lib, however I would like to have them installed into PREFIX/lib/MULTIARCH (where MULTIARCH is an id like x86_64-linux-gnu, or aarch64-linux-gnu).

Passing these macros during configure,

-DCMAKE_INSTALL_FULL_LIBDIR=/$(LLVM_DIR)/lib/$(DEB_HOST_MULTIARCH) \
-DCMAKE_INSTALL_LIBDIR=lib/$(DEB_HOST_MULTIARCH) \

doesn't do anything. The bootstrap build still succeeds, but the default installation locations remain unchanged. Looking at the ninja -v stage2-install call, I also don't see these two macros passed anymore.

Am I missing some LLVM specific macros, or how is the libsubdir configured within LLVM?