This is a rather stupid mangler hack. However, when using prefix data with
llvm, on systems that support -dead_strip (macOS, iOS), the prefix data is
llvm generiously adds .subsections_via_symbols for macho in any case.
Thus we use our trusted mangler to drop the .subsections_via_symbols
line from the assembly.
This ultimately means that for (macOS, llvm), and (iOS, llvm) will not
benefit from -dead_strip. Yet, this patch will allow building ghc on macOS