flang-new: a flag to opt-out from default runtime libraries linking on Windows

At the time flang-new (we use `flang version 20.1.8` for now) links the following libraries when targeting Windows (`x86_64-pc-win32`):

```
lld-link: error: could not open 'clang_rt.builtins.lib': No such file or directory
lld-link: error: could not open 'FortranRuntime.static.lib': No such file or directory
lld-link: error: could not open 'FortranDecimal.static.lib': No such file or directory
```

We run a distributed build system and thus need to have full control over every input, hence we pass flang-rt explicitly (and use our own naming schema).

I do not see an option to turn this behavior off.

Could you, please, add such flag?

At the time we pass three empty libraries to make lld-link work (and it looks like it is finally working).