Check exception specification with __cplusplus.

To allow the compilation with gcc, we can't use the
__has_feature(cxx_noexcept) to detect the default destructor
exception specification, which is noexcept(true) by default
in C++11.  We should use __cplusplus >= 201103L instead.



git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@208465 91177308-0d34-0410-b5e6-96231b3b80d8