Fix AArch64 write return value

buildtest.c

@@ -3,8 +3,12 @@
 
 
 int main() {
-  uint64_t pid = fork();
+  // Test the write syscall
+  intptr_t n = write(STDIO, "Hello\n", 6);
+  if(n != 6) return n;
 
+  // Test the fork syscall
+  uint64_t pid = fork();
   // Print the pid in hex
   char msg[17] = {' '};
   msg[16] = '\n';