Fix and merge libm_gen

[dnezam]

At the time of writing, PR #911 has been open for around 2 years, and it does not seem that anyone is planning to work on merging it any time soon (or exactly knows what that would involve).

The last run of the regression suite seems to be job 2072, which failed due to an unsolved sub-goal. According to @myreen , the change in semantics/semanticPrimitivesScript.sml seems relevant to that proof.
Details about libmGen appear to have been published in chapter 4 of @HeikoBecker PhD thesis (original, archive).