Additional LLVM codegen updates

* drop extra flags from `bin/regression` script
* conditionally implement `Word_mul` in LLVM codegen; as noted earlier, `@llvm.smul.with.overflow.i64` becomes a call to `__mulodi4`, which is provided by the LLVM compiler-rt project (but not by `libgcc`)
* implement `Real_muladd` in LLVM codegen