[openmp] Add OMPT initialization in libomptarget

When loading libomptarget, the init function in libomptarget/src/rtl.cpp
will search for the libomptarget_start_tool function using libdl.
libomptarget_start_tool will pass those OMPT callbacks related to target
constructs to libomptarget

Differential Revision: https://reviews.llvm.org/D99803