Make the famous _S_debug_messages warning go away, by hidining that symbol.

This symbol isn't used anywhere outside libstdc++, thus no bump.

Upstream initially went the samy way, but then implement

Make the famous _S_debug_messages warning go away, by hidining that symbol.

This symbol isn't used anywhere outside libstdc++, thus no bump.

Upstream initially went the samy way, but then implemented a different fix,
which don't work for us. Eventually we should move to whitelisting the list
of symbols exported anyway.

okay miod@, no objections from sthen@; also supported by a few a while ago

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