[patch] libstdc++/64967 disable new std::string for gnu-versioned-namespace

[Jonathan Wakely]

This restores bootstrap for --enable-symvers=gnu-versioned-namespace
by disabling the new std::string ABI.

After GCC 5.1 (probably for GCC 6, rather than 5.2) I will make
another attempt to make gnu-versioned-namespace use the new
std::string only (and bump the soname from libstdc++.so.7 to .so.8)

Tested x86_64-linux, both with and without
--enable-symvers=gnu-versioned-namespace.

There are loads of test failures with gnu-versioned-namespace, but
they are unrelated to the std::string ABI changes (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59256 and
https://gcc.gnu.org/ml/libstdc++/2015-03/msg00058.html for the biggest
causes of test failures).

Committed to trunk.

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