[libc++] Mark usage of _Atomic with __extension__

An upcoming change in Clang will flag _Atomic as being a C11 extension.
To avoid generating this warning in libc++, this commit marks the only
use of _Atomic with the __extension__ extension, which suppresses such
warnings.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@370796 91177308-0d34-0410-b5e6-96231b3b80d8