[PATCH] Add GINV(+VIRT) ASE for MIPSr6

[Tsing]

Hi.
Thanks for the advise on formatting, I will review the code and keep you updated.

>I take it the encoding is correct, as I can't find it in published docs, 
>and the instruction itself is only mentioned once, in the context of SYNC.
>
>How did you verify your change?

The encoding is documented in the I6500 core programming guide and there is 
also a description of the `GINVGT` instruction in the MIPS64 I6500 System Instruction Guide. 
You can find the docs here:
page 176 of https://s3-eu-west-1.amazonaws.com/downloads-mips/documents/MIPS_ Warrior_I6500_ProgrammerGuide_MD01179_P_1.00.pdf

Thanks.
Tsing