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.