[libc][math] Refactor bf16divl to header-only (#181535)

Part of https://github.com/llvm/llvm-project/issues/147386

in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450
Closes #181025

GitOrigin-RevId: 7ad2a632a32b0c6740dff20b117ca8bb52d9cfac
8 files changed