LLVM's OpenMP implementation only supports Windows on x86, and it requires a MASM-compatible assembler to build it. Build UASM for this purpose, for now. (UASM isn't available packaged in debian/ubuntu repos.) In the future, llvm-ml should be able to handle it instead, but it's not currently mature enough. Not including it in the regular build-all.sh for now, as UASM isn't generally available.