C-variadic function definitions are rejected, blocking varargs_without_pattern lint

The rustc varargs_without_pattern lint warns when ... is used as a C-variadic argument in a function definition without a pattern name.

gccrs hard-errors on any c_variadic function definition, even the well-formed unsafe extern "C" form:

#![feature(c_variadic)]
pub unsafe extern "C" fn f(args: ...) {}
// gccrs: only foreign or `unsafe extern "C"` functions may be C-variadic
pub unsafe extern "C" fn g(...) {}
// same error

Since the construct cannot be compiled at all, the lint is moot until gccrs supports C-variadic function definitions.

Start by tracing gccrs handling of c_variadic function definitions and compare it with the valid unsafe extern "C" examples in the issue. Use the provided reproducer to verify that supported C-variadic definitions compile and that varargs_without_pattern can be reached; done means the valid forms are no longer rejected outright.