[OpenMP] [OMPT] [2/8] Implemented a connector for communication of OMPT...

[OpenMP] [OMPT] [2/8] Implemented a connector for communication of OMPT callbacks between libraries.

This is part of a set of patches implementing OMPT target callback support and has been split out of the originally submitted https://reviews.llvm.org/D113728. The overall design can be found in https://rice.app.box.com/s/pf3gix2hs4d4o1aatwir1set05xmjljc

The purpose of this patch is to provide a way to register tool-provided callbacks into libomp when libomptarget is loaded.

Introduced a cmake variable LIBOMPTARGET_OMPT_SUPPORT that can be used to control OMPT target support. It follows host OMPT support, controlled by LIBOMP_HAVE_OMPT_SUPPORT.

Added a connector that can be used to communicate between OMPT implementations in libomp and libomptarget or libomptarget and a plugin.

Added a global constructor in libomptarget that uses the connector to force registration of tool-provided callbacks in libomp. A pair of init and fini functions are provided to libomp as part of the connect process which will be used to register the tool-provided callbacks in libomptarget.

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

Reviewed By: dreachem, jhuber6

Differential Revision: https://reviews.llvm.org/D123572