Add key force_major_gc_count to GC.stat_heap

force_major_gc_count is the number of times the size pool forced major
GC to run.

gc.c

@@ -11005,6 +11005,7 @@ enum gc_stat_heap_sym {
     gc_stat_heap_sym_heap_tomb_slots,
     gc_stat_heap_sym_total_allocated_pages,
     gc_stat_heap_sym_total_freed_pages,
+    gc_stat_heap_sym_force_major_gc_count,
     gc_stat_heap_sym_last
 };
 
@@ -11023,6 +11024,7 @@ setup_gc_stat_heap_symbols(void)
         S(heap_tomb_slots);
         S(total_allocated_pages);
         S(total_freed_pages);
+        S(force_major_gc_count);
 #undef S
     }
 }
@@ -11065,6 +11067,7 @@ gc_stat_heap_internal(int size_pool_idx, VALUE hash_or_sym)
     SET(heap_tomb_slots, SIZE_POOL_TOMB_HEAP(size_pool)->total_slots);
     SET(total_allocated_pages, size_pool->total_allocated_pages);
     SET(total_freed_pages, size_pool->total_freed_pages);
+    SET(force_major_gc_count, size_pool->force_major_gc_count);
 #undef SET
 
     if (!NIL_P(key)) { /* matched key should return above */

test/ruby/test_gc.rb

@@ -160,6 +160,7 @@ def test_stat_heap
       assert_operator stat_heap[:heap_tomb_slots], :>=, 0
       assert_operator stat_heap[:total_allocated_pages], :>=, 0
       assert_operator stat_heap[:total_freed_pages], :>=, 0
+      assert_operator stat_heap[:force_major_gc_count], :>=, 0
     end
 
     GC.stat_heap(0, stat_heap)