Exec format error when stack building Haskell code

This error message suggests that the file in the error message, a cached executable, has been corrupted. If you delete it and rerun your build command, Stack should rebuild it automatically (at least, it did in my tests).

If you get the same error message again or a similar error message for a different file, then that would suggest the problem is more widespread. In that case, I'd suggest uninstalling and reinstalling Stack to see if that clears up the issue. You can uninstall stack by running uninstall-stack in a shell. When the dialog box pops up, make sure you check all the components, including the last two (compilers installed by stack and stack snapshots and configuration), to make sure everything is removed.