[OpenMP] [OMPT] [amdgpu] [5/8] Implemented device init/fini/load callbacks

Added support in the generic plugin to invoke registered callbacks.

Depends on D124070

Patch from John Mellor-Crummey <johnmc@rice.edu>
(With contributions from Dhruva Chakrabarti <Dhruva.Chakrabarti@amd.com>)

Differential Revision: https://reviews.llvm.org/D124652