Passing directories to fopen is not a defined behaviour in C/C++.
A new test case added by https://crrev.com/c/3098189 is trying to
import directories which is expected to fail.

Test however is not passing on some platforms including on S390 Linux
as `fopen` is successful, size gets set to 0 and a (non-existent)
empty file gets returned.

This CL uses `stat` to make sure the path is valid and is
not a directory.

Change-Id: Ibcc762b21145d2198cba07953387a31f39f59300
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3102346Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#76389}