Always build GHCi libs

Since the introduction of -split-sections, using GHCi with the RTS
linker is really slow:

```
$ time (echo :quit | ./inplace/bin/ghc-stage2 --interactive -fexternal-interpreter)
GHCi, version 8.1.20170304: http://www.haskell.org/ghc/  :? for help
Prelude> Leaving GHCi.

real        0m3.793s
```

(when we use `-fexternal-interpreter` it uses the RTS linker by default,
you can make it use the system linker by adding `-dynamic`)

Building the GHCi libs doesn't take much time or space in the GHC build,
but makes things much quicker for people using the RTS linker:

```
$ time (echo :quit | ./inplace/bin/ghc-stage2 --interactive -fexternal-interpreter)
GHCi, version 8.1.20170304: http://www.haskell.org/ghc/  :? for help
Prelude> Leaving GHCi.

real        0m0.285s
```

So I propose that we build and ship them unconditionally.

Test Plan: validate, perf tests above

Reviewers: bgamari, austin, niteria, erikd, Phyx

Reviewed By: bgamari

Subscribers: rwbarton, thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3298

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