Hide atexit.c until it's fixed.

This has the same effect as what the Portfile has been doing all along,
but in a less kludgy manner.

TESTED:
Has the desired effect on the sources.

disabled/README.txt

+The atexit.c source has been temporarily moved here until it can be fixed.
+The Portfile has been removing it all along, so this just makes the local
+build consistent with the port build.  Since it's been disabled since
+its inception in Apr-2022, it clearly hasn't been missed much.  Note that
+the relevant entries in add_symbols.c remain present, but that's also true
+of the way the Portfile was acting.
+
+It would be highly desirable to have reasonable tests for it before
+fixing and reactivating it.
+
+The Portfile block that this replaces (based on the old filename) is:
+
+pre-patch {
+    # until upstream can be fixed, do not include atexit symbols
+    # under certain circumstances, infinite recursive loops can form
+    delete ${worksrcpath}/src/macports_legacy_atexit.c
+}