Fix lto0.test_exceptions_allowed_uncaught failure

This test started failing after #16627.  Prior to that change the
exceptions destructors were called via the following code in JS:

```
   // In Wasm, destructors return 'this' as in ARM
   {{{ makeDynCall('pp', 'destructor') }}}(info.excPtr);
```

I'm not sure why this test only started failing under LTO.  Its seems
like it should be failing under non-LTO too.

Author: sbc100

.circleci/config.yml

@@ -492,6 +492,7 @@ jobs:
           title: "core3+extras"
           test_targets: "
             lto2.test_dylink_syslibs_all
+            lto0.test_exceptions_allowed_uncaught
             core3
             core2g.test_externref
             corez.test_dylink_iostream

system/lib/libcxxabi/src/cxa_exception.h

@@ -24,7 +24,8 @@ namespace __cxxabiv1 {
 struct _LIBCXXABI_HIDDEN __cxa_exception {
   size_t referenceCount;
   std::type_info *exceptionType;
-  void (*exceptionDestructor)(void *);
+  // In wasm, destructors return 'this' as in ARM
+  void* (*exceptionDestructor)(void *);
   uint8_t caught;
   uint8_t rethrown;
   void *adjustedPtr;

system/lib/libcxxabi/src/cxa_exception_emscripten.cpp

@@ -20,7 +20,8 @@
 
 // Define to enable extra debugging on stderr.
 #if EXCEPTIONS_DEBUG
-#define DEBUG printf
+#include "emscripten/console.h"
+#define DEBUG _emscripten_errf
 #else
 #define DEBUG(...)
 #endif
@@ -141,7 +142,7 @@ void __cxa_decrement_exception_refcount(void *thrown_object) _NOEXCEPT {
         assert(exception_header->referenceCount > 0);
         if (std::__libcpp_atomic_add(&exception_header->referenceCount, size_t(-1)) == 0 && !exception_header->rethrown)
         {
-            DEBUG("DEL: %p\n", thrown_object);
+            DEBUG("DEL: %p (dtor=%p)\n", thrown_object, exception_header->exceptionDestructor);
             if (NULL != exception_header->exceptionDestructor)
                 exception_header->exceptionDestructor(thrown_object);
             __cxa_free_exception(thrown_object);