Use __atomic_exchange_n instead of Clang's __sync_swap

Also remove an extra extern "C" from a global variable redeclaration.
This allows building libcxxabi with GCC on my system.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D5604

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@219012 91177308-0d34-0410-b5e6-96231b3b80d8