[libc] [NFC] Split the CMake rules into multiple files.

Summary:
The single file was getting too long to be convenient to navigate. This
patch splits it up two into 4 files one each for header rules,
object rules, library rules, and test rules.

Reviewers: abrachet, alexshap

Subscribers: mgorny, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D78536