re PR libstdc++/33394 (Add test case for Thread race segfault in std::string::append with -O and -s)

	PR libstdc++/33394
	* testsuite/21_strings/basic_string/pthread33394.cc: Add test.

# Auto-generated commit message above this line, original below.
	PR libstdc++/33394
	* testsuite/21_strings/basic_string/pthread33394.cc: Add test.

From-SVN: r221635