Generate the contents of the GHC.Prim interface file automatically
from the list of available PrimOps and various other wired-in things.

Two main benefits from this:

  - There's one fewer places to edit when adding a new primop.

  - It's one less reason to need the interface file parser, and
    now we no longer need the (short-lived) --compile-iface option
    so I've removed it.