Bite the bullet and make GHCi support non-optional in the RTS.  GHC
4.11 should be able to build GHCi without any additional tweaks now.

- the Linker is split into two parts: LinkerBasic.c, containing the
  routines required by the rest of the RTS, and Linker.c, containing
  the linker proper, which is not referred to from the rest of the RTS.
  Only Linker.c requires -ldl, so programs which don't make use of the
  linker (everything except GHC, in other words) won't need -ldl.