[CMake] Disable building libomptarget and add CMake switch

Introduce OPENMP_ENABLE_LIBOMPTARGET which defaults to OFF at the moment.

libomptarget is not yet ready for prime time:
 - Offloading to NVIDIA GPUs is not completed yet (compiler, device RTL)
 - The generic ELF plugin for offloading to the host (meant for testing)
   uses a single instance of the OpenMP runtime (libomp). That is why
   omp_is_initial_device() returns 1 which makes the tests fail.
Because of these reasons, we want to disable building (and testing!)
for release 5.0.

See https://bugs.llvm.org/show_bug.cgi?id=33859

Differential Revision: https://reviews.llvm.org/D35719

llvm-svn: 309115