Force page creation, if major GC doesn't free.

[Bug #20762]

Prior to 51bd816 this conditional would
call `heap_increment` which would always create a new page if one was
unavailable to be resurrected.

This new code uses heap_page_allocate_and_initialize which does nothing
if there are no allocatable slots left in the heap.

The reason we've hit this bug is because we've tried to allocate, run a
major GC, and been unable to free or resurrect a page. In this instance
we should instead use heap_page_allocate_and_initialize_force to force
creation of another page.

gc/default.c

@@ -2164,9 +2164,11 @@ heap_prepare(rb_objspace_t *objspace, rb_size_pool_t *size_pool, rb_heap_t *heap
                         /* Do steps of incremental marking or lazy sweeping. */
                         gc_continue(objspace, size_pool, heap);
 
-                        if (heap->free_pages == NULL &&
-                                !heap_page_allocate_and_initialize(objspace, size_pool, heap)) {
-                            rb_bug("cannot create a new page after major GC");
+                        if (heap->free_pages == NULL) {
+                            heap_page_allocate_and_initialize_force(objspace, size_pool, heap);
+                            if (heap->free_pages == NULL) {
+                                rb_bug("cannot create a new page after major GC");
+                            }
                         }
                     }
                 }