[CUDA] Make __GCC_ATOMIC_XXX_LOCK_FREE macros the same on host/device.

Summary:
This fixes a bug where we were unable to compile the following CUDA
file with libstdc++ (didn't try libc++):

  #include <future>
  void foo() { std::shared_future<int> x; }

The problem is that <future> only defines std::shared_future if
__GCC_ATOMIC_INT_LOCK_FREE > 1.  When we compiled this file for device,
the macro was set to 1, and then the class didn't exist at all.

Reviewers: tra

Subscribers: cfe-commits, jhen

Differential Revision: https://reviews.llvm.org/D24407

llvm-svn: 281089