[OpenMP] [OMPT] [3/8] Implemented callback registration in libomptarget

The purpose of this patch is to have tool-provided callbacks registered
in libomptarget. The overall design document is in
https://rice.app.box.com/s/pf3gix2hs4d4o1aatwir1set05xmjljc

Defined a class OmptDeviceCallbacksTy that will be used by libomptarget
and a plugin for callbacks registered by a tool. Once the callbacks are
registered in libomp, a lookup function is passed to libomptarget that is
used to retrieve the callbacks and register them in libomptarget.

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

Reviewed By: jplehr

Differential Revision: https://reviews.llvm.org/D123974