Retain CAFs in the interpreter

iserv-proxy-interpreter dynamically loads libraries. It must retain CAFs to avoid issues like:

iserv-proxy-interpreter.exe:internal error:Evaluated a CAF that was GC'd!

ghc-lib uses a hacky keepCAFsForGHCi.c file; iserv still uses a custom hs-main C file. We use -fkeep-cafs as it has the same effect.

requested review from @angerman

requested review from @bgamari

@bgamari Do you have merge rights on this repo?

approved this merge request

approved this merge request

mentioned in commit 123190b2

merged

Thanks @bgamari!