When running in -Xshare:dump mode, we need to exit the VM process in a special way

One example is here:
https://github.com/openjdk/jdk/blob/1fdaf252b6375773072e665fd5d4cfb509e98f4c/src/hotspot/share/cds/metaspaceShared.cpp#L567-L571

  // There may be pending VM operations. We have changed some global states
  // (such as vmClasses::_klasses) that may cause these VM operations
  // to fail. For safety, forget these operations and exit the VM directly.
  os::_exit(0);

But there are also other calls to exit the VM when an unrecoverable error happens during CDS archive dumping and loading. Currently, we have ad-hoc calls to os::_exit(0), vm_exit_during_initialization() and vm_direct_exit().

We should change all such calls to a few utility functions (used under different situations) with proper documentation.