message: Add no-op shim for posix_fadvise
diff: @@ -780,6 +780,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  this.write_null(dest)?;
  }
  + "posix_fadvise" => {
  + // fadvise is only informational, we can ignore it.
  + this.write_null(dest)?;
  + }
  +
  "mmap" => {
  // This is a horrible hack, but since the guard page mechanism calls mmap and expects a particular return value, we just give it that value.
  let addr = this.read_scalar(args[0])?.not_undef()?;
  