Remove -ldl for now.

Current story: you don't get any GHCi stuff by default.  To build
GHCi:
  (a) build a compiler with GhcRtsHcOpts+=-optc-DGHCI in build.mk
  (b) use this compiler to bootstrap another compiler, with
      GhcWithInterpreter=YES and GhcHcOpts+=-ldl in build.mk