Do not generate helpers with -betterC

This solves one aspect of [issue 13334](https://issues.dlang.org/show_bug.cgi?id=13334). It sort of takes advantage of the fact that `-betterC` is currently undocumented and poorly defined.

This is not the best possible implementation. Ideally, the strings should be emitted to their own linker section, but I don't know how to do that.