Allow enum to be used for native constants

[dmlloyd]

It should be possible to use an enum to represent some native constants. The driving use case is errno. Since errno values are integers, one naively expects to be able to use them with switch; however, this is disallowed in practice because the values are non-constant as far as Java is concerned. Note that the newer switch statements in Java 21 and later might allow this though.

The constants should be probed lazily as they are today.

Example:

@constant
public enum errno_t {
    EACCES,
    EAGAIN,
    EBADF,
    EINTR,
    EIO,
    //...
}

And then:

    int fd = open(utf8z("/dev/null"), O_RDRW);
    if (fd == -1) {
        switch (errno) {
            case ENOENT -> throw new IOException("No dev null");
            case EINTR -> continue; // restart syscall
            default -> throw new IOException("I/O error");
        }
    }

Internally we could intercept calls to e.g. java.lang.runtime.SwitchBootstraps#enumSwitch and replace them with constant integer switches through a small number of transformations.