update test_stack_overflow for new llvm, which is much harder to trick into hitting a stack overflow

tests/core/stack_overflow.cpp

@@ -1,25 +1,30 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
+#include <emscripten.h>
 
-void recurse(int x);
+void recurse(unsigned int x);
 
-void act(volatile int *a) {
+void act(volatile unsigned int *a) {
   printf("act     %d\n", *a);
-  int b = (int)(intptr_t)(alloca(*a));
+  unsigned int b = (int)(intptr_t)(alloca(*a));
   if (b < *a) *a--;
   recurse(*a);
 }
 
-void recurse(int x) {
+void recurse(volatile unsigned int x) {
   printf("recurse %d\n", x);
-  volatile int a = x;
+  volatile unsigned int a = x;
+  volatile char buffer[1000*1000];
+  buffer[x/2] = 0;
+  buffer[(x-1)/2] = 0;
+  EM_ASM({});
   if (x*x < x) {
     act(&a);
     if (a < x) x = a;
     x--;
-    return;
   }
+  x += buffer[x/2];
   if (x > 0) recurse(x-1);
 }