Update the CheckAtomic CMake module to check for __atomic_fetch_add_4 instead of __atomic_is_lock_free

Summary:
When building with clang, the build fails with:

```
-- Looking for __atomic_is_lock_free in atomic
-- Looking for __atomic_is_lock_free in atomic - not found
CMake Error at cmake/CheckAtomic.cmake:90 (message):
  Host compiler appears to require libatomic, but cannot find it.
Call Stack (most recent call first):
  CMakeLists.txt:75 (include)
```

And the error is:

```
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:7:3: error: conflicting types for '__atomic_is_lock_free'
  CHECK_FUNCTION_EXISTS(void);
  ^
<command line>:1:31: note: expanded from here
#define CHECK_FUNCTION_EXISTS __atomic_is_lock_free
                              ^
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:7:3: note: '__atomic_is_lock_free' is a builtin with type 'int (unsigned long, const volatile void *)'
<command line>:1:31: note: expanded from here
#define CHECK_FUNCTION_EXISTS __atomic_is_lock_free
                              ^
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:17:25: error: too few arguments to function call, expected 2, have 0
  CHECK_FUNCTION_EXISTS();
  ~~~~~~~~~~~~~~~~~~~~~ ^
```

LLVM's CheckAtomic (https://fburl.com/bk14shjt) uses `__atomic_fetch_add_4` so I'm modifying the configs to use it as well to check for the existence of the library.

Reviewed By: yfeldblum

Differential Revision: D19497168

fbshipit-source-id: 64f77487efd16dba49055f6c4cb1cdd0fc4ae6da