[WIP][OpenMP] generic-elf-64bit plugin extension

[mhalk]

Summary

This is a stub, since there are currently no clear objectives on how
we want to proceed with this plugin.
Should we e.g. try to determine certain devices during plugin init?

openmp/libomptarget/plugins-nextgen/generic-elf-64bit/src/rtl.cpp

struct GenELF64DeviceTy : public GenericDeviceTy {
   ~GenELF64DeviceTy() {}
 
   /// Initialize the device, which is a no-op
   Error initImpl(GenericPluginTy &Plugin) override { return Plugin::success(); }
 
   /// Deinitialize the device, which is a no-op
   Error deinitImpl() override { return Plugin::success(); }
 
+  /// See GenericDeviceTy::getComputeUnitKind().
+  std::string getComputeUnitKind() const override { return "generic-64bit"; }
+
   /// Construct the kernel for a specific image on the device.
   Expected<GenericKernelTy &>
   constructKernel(const __tgt_offload_entry &KernelEntry,
                   OMPTgtExecModeFlags ExecMode) override {
     // Allocate and construct the kernel.
     GenELF64KernelTy *GenELF64Kernel =
         Plugin::get().allocate<GenELF64KernelTy>();
     if (!GenELF64Kernel)