[libc] Add a README to the sub-directories under the utils directory.

Also, the source layout document has been updated to reflect the current
layout of the `utils` directory.

Reviewers: PaulkaToast

Differential Revision: https://reviews.llvm.org/D74502

GitOrigin-RevId: 4d812acba61e6306181a830a18d21bcb07b9f8c7
4 files changed