add Mips VI support

[gitoleg]

We have some problems with disassembling and lifting code built for Mips VI release. For example, in this release, some instructions have 3 operands, while they had only 2 in previous releases. So we need to distinguish releases somehow. One possible solution was discussed here: https://github.com/BinaryAnalysisPlatform/bap/pull/767. Finally, we decided to improve our approach to architecture handling. As soon it will be done, we will be able to add a support for Mips VI.