[Bug libstdc++/21334] Lack of Posix compliant thread safety in std::basic_string

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21334
Bug 21334 depends on bug 33394, which changed state.

Bug 33394 Summary: Add test case for Thread race segfault in std::string::append with -O and -s
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33394

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