Make GHC (the library) flexible in the choice of integer library

Summary:
We have more and more users of GHC as a library, for example the
Haskell-to-WebAssembly-compiler https://github.com/tweag/asterius.
These need to make different decisions about various aspects of
code generation than the host compiler, and ideally GHC-the-library
allows them to set the `DynFlags` as needed.

This patch adds a new `DynFlag` that configures which `integer`
library to use. This flag is initialized by `cIntegerLibraryType`
(as before), and is only used in `CorePrep` to decide whether to
use `S#` or not.

The other code paths that were varying based on `cIntegerLibraryType`
are no now longer varying: The trick is to use `integer-wired-in`
as the `-this-unit-id` when compiling either `integer-gmp` or
`integer-simple`.

Test Plan: Validate is happy.

Reviewers: hvr, bgamari

Reviewed By: bgamari

Subscribers: TerrorJack, adamse, simonpj, rwbarton, carter

GHC Trac Issues: #13477

Differential Revision: https://phabricator.haskell.org/D5079

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