Fix space leaks in dynLoadObjs (#16708)

When running the test suite on a GHC built with the `quick` build
flavour, `-fghci-leak-check` noticed some space leaks. Careful
investigation led to `Linker.dynLoadObjs` being the culprit.
Pattern-matching on `PeristentLinkerState` and a dash of `$!` were
sufficient to fix the issue. (ht to mpickering for his suggestions,
which were crucial to discovering a fix)

Fixes #16708.