add `extern "tail"` calling convention Maps to LLVM's `tailcc` and hence inherits all its problems. This calling convention at least compiles on: - `x86_64` - `aarch64` It also compiles on - `s390x` - `loongarch64` But I've been told by target maintainers that this is kind of unintentional, and it's not tested in LLVM. For most backends LLVM does not have support and fails loudly.