Stop running 'dsymutil' on OSX.

I'm not sure that anyone still remembers why this is run, and it emits the
annoying "warning: no debug symbols in executable" warning on OSX. I've traced
its addition to the codebase to two years ago in 196351aa. It's not clear to my
why it was added in this commit, and it appears that ever since then the code
has just gotten moved around.

Until we understand what this is doing and why it should be running, I don't
believe that this should be running by default for all executables that are
created on OSX.

Closes #3495