llvm built fine on my side using my default build script, just adding webassembly. I also changed the triplets as you did in the cmake config.
LLVM (http://llvm.org/):
LLVM version 15.0.7
Optimized build with assertions.
Default target: arm-linux-gnueabihf
Host CPU: cortex-a72
Registered Targets:
arm - ARM
armeb - ARM (big endian)
thumb - Thumb
thumbeb - Thumb (big endian)
wasm32 - WebAssembly 32-bit
wasm64 - WebAssembly 64-bit
Updated extensions uploaded. If this works for firefox, I can update aarch64 llvm as well.