Changes to the RTLD_DEFAULT semantics broke the trick we used to ensure
libc symbols were available to the ghci linker, in OpenBSD 4.0. We can
fix this by linking the ghc binary itself with --export-dynamic on this
system, removing the need for any magic Linker.c games.

GHCi now works on OpenBSD 4.0

Contributed by Eric Mertens <emertens at gmail.com>