Cmm: Promote stack arguments to word size

Smaller than word size integers must be promoted to word size when passed on the stack. While on little endian systems we can get away with writing a small integer to a word size stack slot and read it as a word ignoring the upper bits, on big endian systems a small integer write ends up in the most significant bits and a word size read that ignores the upper bits delivers a random value.

On little endian systems a smaller than word size write to the stack might be more efficient but that decision is system specific and should be done as an optimization in the respective backends.

Fixes #16258 (closed)