[libc] [llvm] [llvm] Add KnownBits implementations for avgFloor and avgCeil (PR #86445)

[Nhat Nguyen via llvm-commits]

changkhothuychung wrote:

> Only 1 minor typo outstanding - thanks

Let me know when you have a chance to review, after that I will merge the PR. Thanks! 

https://github.com/llvm/llvm-project/pull/86445