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

[Owen Anderson via llvm-commits]

resistor wrote:

> Oh, hm, I guess llvm/test/MC/RISCV/target-abi-(in)valid.s should be updated to have initial testing of these code paths?

Added tests

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