fs: fix existsSync for invalid symlink at win32

[pd4d10]

Fixes: #30538

Background

#18618 uses access instead of stat to implement fs.existsSync. Unfortunately, seems the two approaches both have some limitations at Windows:

	Expected behavior of existsSync	stat	access
Invalid symlink	false	✅
File without privileges	true		✅

This PR adds a double check only at win32 platform to fix this issue.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines