Make topenv() static inlined to avoid compiling error when using clang and -O0 mode

Current code only marks top_env as inlined(not static).

If we compile mruby using clang with -O0 mode, the following error would occur:

```
Undefined symbols for architecture x86_64:
  "_top_env", referenced from:
      _mrb_run in libmruby_core.a(vm.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
rake aborted!
```

Marking top_env as static inlined solves this problem.