[libc] Provide fnmatch.h header and fnmatch() function stub. (#219321)

* Provide a POSIX header`<fnmatch.h>` which `fnmatch()` pattern-matching
function (tailored for use on filenames)
  and associated macro.
* Define this as an "experimental" entrypoint on Linux targets
* Implementation is missing at this point, but it might rely (at least
in part) on TBD pattern-matching functionality that is
  being built for `<regex.h>` functions.

Assisted by automated tooling, human-reviewed.

GitOrigin-RevId: f571c5e34a06bc769924ff9100b6f31612279ab0