[llvm] [libc][bazel] Adds missing targets for bf16 functions/tests to bazel files (PR #222071)

[via llvm-commits]

Sukumarsawant wrote:

@s4chinjha can you create separate PRs for each file ( segregating them as public header one, the smoke and the mpfr tests ).
It would be easier that way to review and debug. Keep this one for the public header one

https://github.com/llvm/llvm-project/pull/222071