Fuzzer: make druntime initialization transparent

[JohanEngelen]

To use -fsanitize=fuzzer, the user has to manually initialize druntime, because a user-specified main is not supported. The user has to supply the following code:

extern (C) int LLVMFuzzerTestOneInput(const(ubyte*) data, size_t size)
{
    // D runtime must be initialized, but only once.
    static bool init = false;
    if (!init)
    {
        import core.runtime : rt_init;
        rt_init();
        init = true;
    }

   //... do the actual fuzz testing
}

Perhaps we can add functionality to instead let the user write:

extern (C) int LDC_LLVMFuzzerTestOneInput(const(ubyte*) data, size_t size)
{
   //... do the actual fuzz testing
}

where LLVMFuzzerTestOneInput initializes druntime and calls LDC_LLVMFuzzerTestOneInput.
Another option is to extend libFuzzer functionality or make use of what's already there: https://llvm.org/docs/LibFuzzer.html#startup-initialization

[JohanEngelen]

... or linking with a small LDC library that contains a C module ctor that runs rt_init(). May be useful for other uses too.

[JohanEngelen]

resolved by #2510