[Bazel] Move config settings to platforms

Our current use of `select`s in https://github.com/llvm/llvm-project/blob/main/utils/bazel/llvm-project-overlay/llvm/config.bzl is outdated and inflexible.

Move this to a `config/BUILD.bazel` so that all the individual options are configurable and integrate well with platforms.

See also: #125681