Fix C++ Atomic support check (#2126)

Summary:
Some platforms need libatomic linked for sub-word operations while others need it for super-word operations. Let's make sure we test for all.

Relates to #1907

Pull Request resolved: #2126

Reviewed By: Gownta

Differential Revision: D52975209

Pulled By: yfeldblum

fbshipit-source-id: 2c8781f817d6cc59fb8c74c1d13812e584187b47

CMake/folly-deps.cmake

@@ -187,9 +187,13 @@ message(STATUS "Setting FOLLY_HAVE_DWARF: ${FOLLY_HAVE_DWARF}")
 check_cxx_source_compiles("
   #include <atomic>
   int main(int argc, char** argv) {
+    std::atomic<uint8_t> a1;
+    std::atomic<uint16_t> a2;
+    std::atomic<uint32_t> a4;
+    std::atomic<uint64_t> a8;
     struct Test { bool val; };
     std::atomic<Test> s;
-    return static_cast<int>(s.is_lock_free());
+    return a1++ + a2++ + a4++ + a8++ + unsigned(s.is_lock_free());
   }"
   FOLLY_CPP_ATOMIC_BUILTIN
 )
@@ -200,9 +204,13 @@ if(NOT FOLLY_CPP_ATOMIC_BUILTIN)
   check_cxx_source_compiles("
     #include <atomic>
     int main(int argc, char** argv) {
+      std::atomic<uint8_t> a1;
+      std::atomic<uint16_t> a2;
+      std::atomic<uint32_t> a4;
+      std::atomic<uint64_t> a8;
       struct Test { bool val; };
-      std::atomic<Test> s2;
-      return static_cast<int>(s2.is_lock_free());
+      std::atomic<Test> s;
+      return a1++ + a2++ + a4++ + a8++ + unsigned(s.is_lock_free());
     }"
     FOLLY_CPP_ATOMIC_WITH_LIBATOMIC
   )