gpython: Display build information and support some builtin attribute

Likewise, PyPy3, if we can display build information such as commit information, go version.
it will be awesome.

``` python
Python 3.5.3 (fdd60ed87e941677e8ea11acf9f1819466521bf2, Apr 26 2018, 01:23:42)
[PyPy 6.0.0 with GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
```

Also, we need to implement `copyright`, `credits`and `license` as a built-in attribute.

- [ ] copyright
- [ ] credits
- [ ] license
- [x] build info