ext/objspace: remove OS.after_gc_start_hook= and move internal test

* ext/objspace/gc_hook.c: remove this file
* ext/-test-/tracepoint/gc_hook.c: new filename for above
* ext/objspace/objspace.c: remove ObjectSpace.after_gc_start_hook=
* test/objspace/test_objspace.rb: remove test
* test/-ext-/tracepoint/test_tracepoint.rb: add above test for
  tracepoint re-entry

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44007 b2dd03c8-39d4-4d8f-98ff-823fe69b080e