RFR: 8334489: Add function os::used_memory

[Johan Sjölen]

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

A naïve implementation is:

```c++
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.

```c++
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 wrong
  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. The default implementation will use the naïve implementation.

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Commit messages:
 - Introduce os::used_memory

Changes: https://git.openjdk.org/jdk/pull/19772/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19772&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334489
  Stats: 23 lines in 5 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19772.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19772/head:pull/19772

PR: https://git.openjdk.org/jdk/pull/19772