[OpenMP][libomptarget] Add minor fixes to NextGen plugins

List of fixes:
  - omptarget_device_environment symbol is not mandatory in device images
  - Do not synchronize in ~AsyncInfoWrapperTy() if the async info's queue is null
  - GenericDeviceResourceRef's create() and destroy() require the device as parameter

Differential Revision: https://reviews.llvm.org/D138619