[3.12] gh-116510: Fix a crash due to shared immortal interned strings. (gh-124541)

Fix a crash caused by immortal interned strings being shared between
sub-interpreters that use basic single-phase init.  In that case, the string
can be used by an interpreter that outlives the interpreter that created and
interned it.  For interpreters that share obmalloc state, also share the
interned dict with the main interpreter.