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

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s4chinjha wrote:

i tested the bf16 smoke + MPFR tests locally and they passed, but the MPFR tests failed to build in ci because they were missing the same deps. this follow-up commit adds those missing deps, and i've re-tested them locally and they pass.

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