Force findPtr to be included in the binary

Summary: A better alternative to D4657.

Test Plan:
```
cd testsuite/tests/codeGen/should_run
../../../../inplace/bin/ghc-stage2 -debug cgrun001
nm cgrun001 | grep findPtr
```

Reviewers: bgamari, Phyx, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4683