Platform-specific function ABIs are not validated

[jonas-schievink]

This code builds (and runs) fine on stable Rust when targeting x86_64-unknown-linux-gnu:

extern "aapcs" fn f() {}

fn main() {
    f();
}

Other nonsensical ABIs are also accepted, including win64, avr-interrupt (with feature gate). I expected all of these to result in a compilation error, since they are unsupported on x86_64-unknown-linux-gnu.

cc #57182 #86231

[nagisa]

Duplicate of #57182