Make async_kill_with_syscallbuf2 run long enough on 32-bit systems

On x86-32 it exits after 2^32 iterations of
```
   0x08048485 <+117>:	addl   $0x1,-0xc(%ebp)
   0x08048489 <+121>:	cmpl   $0xffffffff,-0xc(%ebp)
   0x0804848d <+125>:	jne    0x8048485 <main+117>
```
which often completes in less than one second on my Arrow Lake system...
So do 2^64 iterations on x86-32 like we do on the 64-bit platforms.

Author: rocallahan

src/test/async_kill_with_syscallbuf2.c

@@ -3,7 +3,7 @@
 #include "util.h"
 
 int main(void) {
-  unsigned long iteration = 0;
+  uint64_t iteration = 0;
   int pipe_fds[2];
   pipe(pipe_fds);
 
@@ -16,7 +16,7 @@ int main(void) {
   // Write again.
   write(pipe_fds[1], "hi\n", 3);
 
-  while (iteration < ULONG_MAX) {
+  while (iteration < UINT64_MAX) {
     iteration += 1;
     // Don't let the compiler delete this loop.
     asm("" : : : "memory");