variadic functions: remove list of supported ABIs from error
I think this list is problematic for multiple reasons:
- It is bound to go out-of-date as it is in a very different place from where we actually define which functions support varagrs (`fn supports_varargs`).
- Many of the ABIs we list only work on some targets; it makes no sense to mention "aapcs" as a possible ABI when building for x86_64. (This led to a lot of confusion in https://github.com/rust-lang/rust/issues/110505 where the author thought they should use "cdecl" and then were promptly told that "cdecl" is not a legal ABI on their target.)
- Typically, when the programmer wrote `extern "foobar"`, it is because they need the "foobar" ABI. It is of little use to tell them that there are other ABIs with which varargs would work.
Cc ``@workingjubilee``
fe54c3a5eb