Avoid g++.dg/torture/pr106922.C FAIL with the pre-C++11 ABI

The following forces the g++.dg/torture/pr106922.C testcase to use
the C++11 libstdc++ ABI and checks whether that worked.

gcc/testsuite/
	* g++.dg/torture/pr106922.C: Force _GLIBCXX_USE_CXX11_ABI to 1.