WASI-SDK readlink() has invalid error handling

I am posting this as an advisory notice so people can find it if they search for readlink(), I don't need it fixed.

POSIX says that if readlink() is applied to an actual file, not a link, it should return -1 and set errno to EINVAL (https://pubs.opengroup.org/onlinepubs/9699919799.2016edition/functions/readlink.html). In Wasi-SDK, it returns 3 and places the 3-character string "-22" in the buf, the second parameter to readlink().

This is a minimal test program to check the invalid behavior (run `touch testfile` and then the test program):
```

#include
#include
#include

int main(void)
{
const char *pathname = "testfile";
char buf[_POSIX_PATH_MAX];
ssize_t len;

if ((len = readlink (pathname, buf, sizeof (buf) - 1)) != -1)
{
buf[len] = 0;
fprintf(stderr, "DBG -- readlink() returned %zd\n", len);
fprintf(stderr, "and put %s in return buffer\n", buf);
return 1;
} else {
printf("readlink() returned correct error\n");
return 0;
}
}

```

Start by compiling and running the minimal C program with WASI-SDK after creating testfile, then compare readlink() behavior with the POSIX specification linked in the issue. The report is advisory and explicitly says no fix is needed, so a completed change is not defined by the payload.

Written by the indexing model from the issue text.