[OpenMP][OMPT] Add OMPT support for `generic-elf-64bit` plugin

Fixes: https://github.com/llvm/llvm-project/issues/64487
Connect OMPT during plugin initialization and enable corresponding tests.
Avoid linking OMPT when corresponding support is disabled.

Depends on D158542

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D158543