libiconvReal: implement ABI compatibility on Darwin

This commit prepares libiconvReal to replace darwin.libiconv, allowing
it to be used with binary derivations that patch out references to the
system libiconv with one from nixpkgs.

Apple’s libiconv is based on GNU libiconv 1.11 (the last version before
it switched to LGPLv3+). Any newer releases by Apple appear to be build
system tweaks. The core sources are barely updated. This means that
Darwin users won’t get any fixes from upstream updates, and maintaining
darwin.libiconv requires dealing with a separate and different build
system (because Apple now builds it with Xcode). Fortunately, it is
possible to build upstream libiconv in a way that is compatible with
Apple’s distribution of it.

There are two things that need to happen to produce an ABI-compatible
build of libiconv:

* Existing symbols need to be exported with the `iconv_` prefix instead
  of the `libiconv_` prefix. New symbols can have the `libiconv` prefix,
  and one symbol in Apple’s distribution does, but older ones must have
  the older prefix; and
* Reexport `libcharset.dylib` from `libiconv.dylib`. This is explained
  by Apple as the result of a bug in their transition to an Xcode-based
  build system.

Both these these are doable and have been done by this commit. I have
tested it with building GHC, which downloads a binary distribution as
part of its bootstrap and replaces references to the system libiconv
with darwin.libiconv. Using this patch, libiconvReal is able to work
without any changes to the GHC derivation.

Note that this patch does not actually deprecate or remove
darwin.libiconv yet. That will be done in a future patch after Darwin
support is added for aliases and deprecating packages in the `darwin`
attrset.

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