[OpenMP] Introduce the keepAlive function into the old device RT

[ye-luo]

LGTM

openmp/libomptarget/deviceRTLs/common/src/support.cu

void __kmp_invoke_microtask(kmp_int32 global_tid, kmp_int32 bound_tid, void *fn,
   switch (nargs) {
 #include "common/generated_microtask_cases.gen"
   default:
     printf("Too many arguments in kmp_invoke_microtask, aborting execution.\n");
     __builtin_trap();
   }
 }
 
+namespace _OMP {
+/// Helper to keep code alive without introducing a performance penalty.
+__attribute__((used, weak, optnone)) void keepAlive() {
+  __kmpc_get_hardware_thread_id_in_block();
+  __kmpc_get_hardware_num_threads_in_block();
+  __kmpc_barrier_simple_spmd(nullptr, 0);
+  __kmpc_barrier_simple_generic(nullptr, 0);
+}
+} // namespace _OMP
+
 #pragma omp end declare target