Fix execute permissions for flatc binary extracted from PAR files (#18738)

Summary:

When binaries are extracted from PAR files via `importlib.resources.as_file()`,
execute permissions are not preserved, causing PermissionError when trying to
run them.

This change ensures the flatc binary has execute permissions after extraction.

Files fixed:
- `executorch/exir/_serialize/_flatbuffer.py`: `_run_flatc` - ensure flatc binary has execute permissions

Differential Revision: D99780508

Author: navsud

exir/_serialize/_flatbuffer.py

@@ -252,6 +252,14 @@ def _run_flatc(args: Sequence[str]) -> None:
     if flatc_resource.is_file():
         # Use the provided flatc binary.
         with importlib.resources.as_file(flatc_resource) as flatc_path:
+            # Ensure the binary has execute permissions (needed for PAR files)
+            import stat
+
+            current_mode = flatc_path.stat().st_mode
+            if not (current_mode & stat.S_IXUSR):
+                flatc_path.chmod(
+                    current_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH
+                )
             subprocess.run([flatc_path] + list(args), check=True)
     else:
         # Expect the `flatc` tool to be on the system path or set as an env var.