Support freeing the lowest memory address page

This should help fix the following flaky test:

```
  1) Failure:
TestProcess#test_warmup_frees_pages [test/ruby/test_process.rb:2751]:
<0> expected but was
<1>.
```

gc.c

@@ -2058,7 +2058,7 @@ heap_pages_free_unused_pages(rb_objspace_t *objspace)
     }
 
     if (has_pages_in_tomb_heap) {
-        for (i = j = 1; j < heap_allocated_pages; i++) {
+        for (i = j = 0; j < heap_allocated_pages; i++) {
             struct heap_page *page = heap_pages_sorted[i];
 
             if (page->flags.in_tomb && page->free_slots == page->total_slots) {
@@ -2078,6 +2078,11 @@ heap_pages_free_unused_pages(rb_objspace_t *objspace)
         GC_ASSERT(himem <= heap_pages_himem);
         heap_pages_himem = himem;
 
+        struct heap_page *lopage = heap_pages_sorted[0];
+        uintptr_t lomem = (uintptr_t)lopage->start;
+        GC_ASSERT(lomem >= heap_pages_lomem);
+        heap_pages_lomem = lomem;
+
         GC_ASSERT(j == heap_allocated_pages);
     }
 }