explicit_extern_abis

The tracking issue for this feature is: #134986

Disallow extern without an explicit ABI. We should write extern "C" (or another ABI) instead of just extern.

By making the ABI explicit, it becomes much clearer that "C" is just one of the possible choices, rather than the "standard" way for external functions. Removing the default makes it easier to add a new ABI on equal footing as "C".

#![allow(unused)]
#![feature(explicit_extern_abis)]

fn main() {
extern fn function1() {}  // ERROR `extern` declarations without an explicit ABI
                          // are disallowed

extern "C" fn function2() {} // compiles

extern "aapcs" fn function3() {} // compiles
}