Finish GC before calling gc_set_initial_pages

If we are during incremental sweeping when calling gc_set_initial_pages
there is an assertion error. The following patch will artificially
produce the bug:

```
diff --git a/gc.c b/gc.c
index c3157dbe2c..d7282cf8f0 100644
--- a/gc.c
+++ b/gc.c
@@ -404,7 +404,7 @@ int ruby_rgengc_debug;
  * 5: show all references
  */
 #ifndef RGENGC_CHECK_MODE
-#define RGENGC_CHECK_MODE  0
+#define RGENGC_CHECK_MODE  1
 #endif
 // Note: using RUBY_ASSERT_WHEN() extend a macro in expr (info by nobu).
@@ -10821,6 +10821,10 @@ gc_set_initial_pages(void)
 void
 ruby_gc_set_params(void)
 {
+    for (int i = 0; i < 10000; i++) {
+        rb_ary_new();
+    }
+
     /* RUBY_GC_HEAP_FREE_SLOTS */
     if (get_envparam_size("RUBY_GC_HEAP_FREE_SLOTS", &gc_params.heap_free_slots, 0)) {
        /* ok */
```

The crash looks like:

```
Assertion Failed: ../gc.c:2038:heap_add_page:!(heap == heap_eden && heap->sweeping_page)
```

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