[patch] libstdc++/66441 Fix wstring_convert when generating BOM

[Jonathan Wakely]

If the codecvt facet generates a BOM on every call to out() then we
need to ensure there is enough capacity in the output string for the
entire result, otherwise we loop and insert more than one BOM into the
result.

Tested powerpc64-linux and powerpc64-linux.

Committing to trunk and gcc-5-branch.
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