[integration 0/1] RISC-V: Add CSRs and opcodes of T-HEAD XUANTIE CPUs

[Lifang Xia]

Hi Nelson,
Thanks for reveiwing these patches.
That's my fault that I'm not checking these patches in
binutils-integration-branch. I thought there are not much changes from
master. Sorry. :(

I re-write the patches, merge them to one patch here.
The extended solutions is suitable for adding CSRs and opcodes of T-HEAD
XUANTIE CPUs.
As you metioned in the previous e-mail, I agree with that it is not a
good solution for vendors. Is there any discussions about that?

Thanks!
Lifang Xia