[flang][runtime] Replace recursion with iterative work queue

Recursion, both direct and indirect, prevents accurate stack size
calculation at link time for GPU device code.  Restructure these
recursive (often mutually so) routines in the Fortran runtime
with new implementations based on an iterative work queue with
suspendable/resumable work tickets: Assign, Initialize, initializeClone,
Finalize, and Destroy.  Descriptor-based I/O is now also supported.

Note that derived type FINAL subroutine calls, defined assignments,
and defined I/O procedures all perform callbacks into user code,
which may well reenter the runtime library.  This kind of recursion
is not handled by this change, although it may be possible to do so
in the future using thread-local work queues.

The effects of this restructuring on CPU performance are yet to be
measured.

Author: klausler

flang-rt/include/flang-rt/runtime/environment.h

@@ -63,6 +63,7 @@ struct ExecutionEnvironment {
   bool noStopMessage{false}; // NO_STOP_MESSAGE=1 inhibits "Fortran STOP"
   bool defaultUTF8{false}; // DEFAULT_UTF8
   bool checkPointerDeallocation{true}; // FORT_CHECK_POINTER_DEALLOCATION
+  int internalDebugging{0}; // FLANG_RT_DEBUG
 
   // CUDA related variables
   std::size_t cudaStackLimit{0}; // ACC_OFFLOAD_STACK_SIZE