[libc][bazel] Define libc namespace in a separate file and drop the `release` flag (#68563)

The `release` flag is misleading and its semantics are not well defined.
Originally this was meant to allow for different `LIBC_NAMESPACE`
depending on whether the code was considered stabled and released or
unstable. It appears that we may have a canary environment that is
neither released or dev. As a consequence we move the `LIBC_NAMESPACE`
definition to its own file and each environment can override this file
with whatever makes sense.