[Bug libstdc++/55394] Using call_once without -lpthread compiles without warning

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55394
Bug 55394 depends on bug 84323, which changed state.

Bug 84323 Summary: call_once uses TLS even when once_flag is set
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84323

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED