[libc][bazel] Prevent LIBC_NAMESPACE leakeage (#70455)

Right now `LIBC_NAMESPACE` definition is added to all rules that depend on any libc function. This have far-reaching effects which prevents llvm-libc development when the installed libc is itself llvm-libc.

This PR removes the `:libc_root` target entirely and consistently uses the provided libc BUILD actions for compilation and testing. This makes sure that we don't leak `LIBC_NAMESPACE` to our clients.