FS permission handling seems odd

The following test code runs perfectly fine when compiled with clang and ran, but when ran with emscripten the `ftruncate` operation throws a `Permission error` instead of going through as expected.

```c
#include
#include

#include
#include
#include

int main() {
int fd = 0;
char filename[] = "XXXXXX";

// Test
assert((fd = mkstemp(filename)) >= 0);
assert(chmod(filename, 0) >= 0);
assert(ftruncate(fd, 69) >= 0);

// Cleanup
assert(close(fd) >= 0);
assert(remove(filename) >= 0);

printf("ok\n");
return 0;
}
```
Ran with
- `clang main.c -o testfs && ./testfs`
- `emcc main.c -s ENVIRONMENT=node -s MAIN_MODULE -o out.js && node out.js`

### Current behavior
Clang build prints ok, emcc build fails (assertion error at ftruncate)

### Expected behavior
Both should print "ok"

### Environment
OS: Arch Linux
clang version 12.0.1
emscripten version 2.0.26 (with clang 13.0.0)
node version 16.4.2