[3.14] gh-146041: Avoid lock in sys.intern() for already interned strings (gh-146072) (#146390)

Fix free-threading scaling bottleneck in sys.intern and `PyObject_SetAttr` by
avoiding the interpreter-wide lock when the string is already interned and
immortalized.

(cherry picked from commit 60093096ba62110151d822b072a01061876e9404)