Add -fexternal-dynamic-refs

Summary:
The `-dynamic` flag does two things:

* In the code generator, it generates code designed to link against
  external shared libraries.  References outside of the current module
  go through platform-specific indirection tables (e.g. the GOT on ELF).

* It enables a "way", which changes which hi files we look
  for (`Foo.dyn_hi`) and which libraries we link against.

Some specialised applications want the first of these without the
second. (I could go into detail here but it's probably not all that
important).

This diff splits out the code-generation effects of `-dynamic` from the
"way" parts of its behaviour, via a new flag `-fexternal-dynamic-refs`.

Test Plan: validate

Reviewers: niteria, bgamari, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4477

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