Cross-compilation of the runtime isn’t really supported right now, this is an unfortunate consequence of the runtime growing within the project itself without distinction.
This is something I’m trying to fix with [RFC] Restructuring of the MLIR repo by introducing a runtime/ folder that would be compiled for the “target” instead of the “host” (where the compiler runs).
The only way right now (I think) is to build separately the project for ARM (basically cross-compile the compiler itself for a ARM “host”) and use the objects from there.