[CUDA] Ignore __CLANG_ATOMIC_LLONG_LOCK_FREE on i386

The default host CPU for an i386 triple is typically at least an i586,
which has cmpxchg8b (Clang feature, "cx8"). Therefore,
`__CLANG_ATOMIC_LLONG_LOCK_FREE` is 2 on the host, but the value should
be 1 for the device.

Also, grep for `__CLANG_ATOMIC_*` instead of `__GCC_ATOMIC_*`. The CLANG
macros are always emitted, but the GCC macros are omitted for the
*-windows-msvc targets. The `__GCC_HAVE_SYNC_COMPARE_AND_SWAP` macro
always has GCC in its name, not CLANG, however.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D127465