When you configure GCC using the configure script, it will construct the file Makefile from the template file Makefile.in. When it does this, it can incorporate makefile fragments from the config directory. These are used to set Makefile parameters that are not amenable to being calculated by autoconf. The list of fragments to incorporate is set by config.gcc; See System Config.
Fragments are named either t-target or x-host, depending on whether they are relevant to configuring GCC to produce code for a particular target, or to configuring GCC to run on a particular host. Here target and host are mnemonics which usually have some relationship to the canonical system name, but no formal connection.
If these files do not exist, it means nothing needs to be added for a given target or host. Most targets need a few t-target fragments, but needing x-host fragments is rare.