Make GHC.Cmm.Switch magic constants tunable

Currently we have some magic constants at https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Cmm/Switch.hs#L53. I propose making them tunable via GHC options, or at the very least, platform-dependent.

The current jump table size looks too small for wasm. Most engines have a much higher max br_table size limit, so bumping this for wasm would allow more compact code to be generated for big Cmm switches.

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