builders/writeHaskell: build with threaded GHC runtime

Some haskell code starts silently hanging when not built with a
threaded runtime, so let’s assume people using `writeHaskell` don’t
care about micro-optimizations like this and do the expected thing.

Some architectures don’t support a threaded runtime, for these we
provide the `threadedRuntime` option to turn it off (it should fail at
build time in that case, easy to detect).

If somebody already passed `"-threaded"` before via ghcArgs, this
will not add the flag a second time. Thus it’s backward-compatible in
this regard.

I tested out both branches (with `-threaded` set and not set before),
on an example I had where the runtime would hang when not compiled
with `-threaded`.

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