[llvm] [RISCV][CHERIoT] Define ABI_CHERIOT. (PR #192929)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Hm, I distinctly remember writing this week, prompted by your latest ping and Craig's comment bringing it to the top of my inbox, what I'm about to write again, but it's nowhere to be found. I wonder what happened there...

This needs rebasing, and the commit message still references a separate bare-metal ABI.

https://github.com/llvm/llvm-project/pull/192929