Changing compilers/sysroots for LLVM_RUNTIME_TARGETS

Hi all, apologies in advance if this isn’t the right place for this kind of question.

I maintain a custom LLVM cross-compile toolchain (for Linux). Right now I build clang, lldb etc for the host, and I have several custom GCC cross-compilers that I use to build compiler-rt and openmp, as external projects. I can then use clang with the -gcc-toolchain flag to specify the sysroot and everything works great.

Building the runtime libraries individually for each target is onerous and I’d rather do something that’s well-supported. Is there a way to specify custom sysroots/extra flags when building the runtimes with LLVM_RUNTIME_TARGETS, so I can do a standard runtimes build? Perhaps there is some trick I am missing.

Thanks for any help!
Caleb