[libc] customizable namespace 1/4 (#65321)

This implements the first step of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Make required Bazel and CMake preparatory changes which define the var
for the customizable namespace and use it (pass it as a compile option
-DLIBC_NAMESPACE=<...>).

Differential Revision: https://reviews.llvm.org/D159112