Ensure that GC is disabled during compaction

Various things can cause GC to occur when compaction is running, for
example resizing the object identity map:

```
    frame #24: 0x000000010c784a10 ruby`gc_grey [inlined] push_mark_stack(stack=<unavailable>, data=<unavailable>) at gc.c:4311:42
    frame #25: 0x000000010c7849ff ruby`gc_grey(objspace=0x00007fc56c804400, obj=140485906037400) at gc.c:4907
    frame #26: 0x000000010c78f881 ruby`gc_start at gc.c:6464:8
    frame #27: 0x000000010c78f5d1 ruby`gc_start [inlined] gc_marks_start(objspace=0x00007fc56c804400, full_mark=<unavailable>) at gc.c:6009
    frame #28: 0x000000010c78f3c0 ruby`gc_start at gc.c:6291
    frame #29: 0x000000010c78f399 ruby`gc_start(objspace=0x00007fc56c804400, reason=<unavailable>) at gc.c:7104
    frame #30: 0x000000010c78930c ruby`objspace_xmalloc0 [inlined] objspace_malloc_fixup(objspace=<unavailable>, mem=0x000000011372a000, size=<unavailable>) at gc.c:9665:5
    frame #31: 0x000000010c7892f5 ruby`objspace_xmalloc0(objspace=0x00007fc56c804400, size=12582912) at gc.c:9707
    frame #32: 0x000000010c89bc13 ruby`st_init_table_with_size(type=<unavailable>, size=<unavailable>) at st.c:605:39
    frame #33: 0x000000010c89c5e2 ruby`rebuild_table_if_necessary [inlined] rebuild_table(tab=0x00007fc56c40b250) at st.c:780:19
    frame #34: 0x000000010c89c5ac ruby`rebuild_table_if_necessary(tab=0x00007fc56c40b250) at st.c:1142
    frame #35: 0x000000010c89c379 ruby`st_insert(tab=0x00007fc56c40b250, key=140486132605040, value=140485922918920) at st.c:1161:5
    frame #36: 0x000000010c794a16 ruby`gc_compact_heap [inlined] gc_move(objspace=0x00007fc56c804400, scan=<unavailable>, free=<unavailable>, moved_list=140485922918960) at gc.c:7441:9
    frame #37: 0x000000010c794917 ruby`gc_compact_heap(objspace=0x00007fc56c804400, comparator=<unavailable>) at gc.c:7695
    frame #38: 0x000000010c79410d ruby`gc_compact [inlined] gc_compact_after_gc(objspace=0x00007fc56c804400, use_toward_empty=1, use_double_pages=<unavailable>, use_verifier=1) at gc.c:0:22
```

We *definitely* need the heap to be in a consistent state during
compaction, so this commit sets the current state to "during_gc" so that
nothing will trigger a GC until the heap finishes compacting.

This fixes the bug we saw when running the tests for https://github.com/ruby/ruby/pull/2264

gc.c

@@ -8260,7 +8260,9 @@ gc_compact(rb_objspace_t *objspace, int use_toward_empty, int use_double_pages,
         /* pin objects referenced by maybe pointers */
         rb_gc();
         /* compact */
+        during_gc = TRUE;
         gc_compact_after_gc(objspace, use_toward_empty, use_double_pages, TRUE);
+        during_gc = FALSE;
     }
     objspace->flags.during_compacting = FALSE;
     return gc_compact_stats(objspace);