system: Add 'locale-libcs' field.

system: Add 'locale-libcs' field.

* gnu/system/locale.scm (localedef-command)[maybe-version-directory]:
  New procedure.
  Use it.
  (locale-directory): Rename to...
  (single-locale-directory): ... this.  Check the version of LIBC to
  determine whether to create a "X.Y" sub-directory or to make it a
  symlink to ".".  Add the version number in the derivation name.
  (locale-directory): New procedure.
  (%default-locale-libcs): New variable.
* gnu/system.scm (<operating-system>)[locale-libcs]: New field.
  (operating-system-locale-directory): Pass it to 'locale-directory'.
* doc/guix.texi (operating-system Reference): Document 'locale-libcs'.
  (Locales)[Locale Data Compatibility Considerations]: New section.