mruby-strip doesn't need mrbgems

Reduces executable size by more than 50% (409->171 KB).
Refactor obj_free() in gc.c
Merge pull request #2509 from suzukaze/refactor-gc

Refactor obj_free() in gc.c
Merge pull request #2510 from cremno/mruby-strip-use-mrb_open_core

mruby-strip doesn't need mrbgems