[libc] Bazel overlay for libc

This patch provides a draft overlay to support compilation of llvm libc with Bazel.

Tested on linux x86-64 with
```
cd git/llvm-project/utils/bazel
bazelisk-linux-amd64 build --sandbox_base=/dev/shm --config=generic_clang @llvm-project//libc:all
```

Differential Revision: https://reviews.llvm.org/D114712