RISC-V implementation files have inaccurate copyright date or missing copyright header.

[phillipstanleymarbell]

The RISC-V implementation files in 'sim/' have inaccurate copyright date (should be 2017-2018, Zhengyang Gu (author)) and several are missing the copyright header / block.

(cc: @Z-Gu)

[phillipstanleymarbell]

Should be fixed in #132.