Add function os::used_memory

A function os::used_memory is needed for GC ergonomics.

A naïve implementation is:

```
julong os::used_memory() {
  return os::physical_memory() - os::available_memory();
}
```

This function does not work well in a containerized environment, as the amount of physical memory may change. To understand why, we must look under the hood.

```
julong os::used_memory() {
  return os::physical_memory() - os::available_memory();
}

// can be expanded into

julong os::used_memory() {
  // The os::physical_memory() call
  julong mem_physical1 = OSContainer::memory_limit_in_bytes();
  // The os::available_memory() call
  julong mem_used = OSContainer::memory_usage_in_bytes();
  julong mem_physical2 = OSContainer::memory_limit_in_bytes();
  
  // Uh-oh: mem_physical1 may differ from mem_physical2 at this point
  // That means that this number is unreliable
  return mem_physical1 - (mem_physical2 - mem_used);
}
```

The fix is to expose OSContainer::memory_usage_in_bytes if it's available, as this call does not depend on OSContainer::memory_limit_in_bytes.