Fix and merge libm_gen

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](https://cakeml.org/regression.cgi/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](https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/34919/1/main.pdf), [archive](http://web.archive.org/web/20240423152333/https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/34919/1/main.pdf)).

Start with PR #911 and regression job 2072 to understand the incomplete libm_gen merge and its unsolved sub-goal. Read semantics/semanticPrimitivesScript.sml and chapter 4 of the referenced thesis for context. Done means the regression suite passes and the libm_gen changes are merged.

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