gh-78214: marshal: Stabilize FLAG_REF usage (GH-8226)

Use FLAG_REF always for interned strings.

Refcounts of interned string is very unstable.
When compiling same source, refcounts of interned string in the output may be 1 or >1.
It makes FLAG_REF usage unstable.

To help reproducible build, use FLAG_REF for interned string even if refcnt(obj)==1.