gc.c: simple array for wmap

* gc.c (wmap_finalize, wmap_aset_update): use simple malloced array
  instead of T_ARRAY, to reduce GC pressure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44119 b2dd03c8-39d4-4d8f-98ff-823fe69b080e