We need to define this function when compiling with clang -m32 -ftrapv,
since otherwise we get link errors, since apparently some versions
of libclang_rt.builtins don't define a version of it that works? Or
clang doesn't know to look for it?

This definition is taken from the LLVM source at
  https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/builtins/mulodi4.c

I've also included the license (dual BSD-ish/MIT-ish).