cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/media/tatokis/Lain/lainux/ps2clang/sysprefix -DLLVM_TARGETS_TO_BUILD="Mips" -DLLVM_INSTALL_BINUTILS_SYMLINKS=ON -DLLVM_ENABLE_THREADS=OFF -DLLVM_TARGET_ARCH="Mips" -DLLVM_DEFAULT_TARGET_TRIPLE=mips64r5900el-ps2-elf ..

I realise "mips64r5900el" is not technically a valid architecture, but recent convention for building GCC in the PS2 development toolchain uses this triple, and the LLVM manual states "When using Clang, it’s important that you choose the triple to be identical to the GCC triple and the sysroot." (https://llvm.org/docs/HowToCrossCompileLLVM.html#configuring-cmake)