Summary:
Multiple tests use the same source C file.
GHC was previously writing the resulting .o files
in the same folder as the source. When these tests
are run in parallel, sometimes one of the calls would
use the partial .o file and throw an error.

The .o files are moved into different folders with this
change.

Test Plan: make test -C testsuite/tests/ghci/linking/dyn

Reviewers: thomie, austin, bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1449