Make atomics extern "C" again.

Pawel changed the implementation but I see no reason to make those available only from C++, so it must be an oversight.
Fixes building Haiku on Haiku which otherwise hits a mismatch in build compatibility headers.

headers/os/support/SupportDefs.h

@@ -217,6 +217,9 @@ extern void*	get_stack_frame(void);
 
 #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 static inline void
 atomic_set(int32* value, int32 newValue)
@@ -319,6 +322,10 @@ atomic_get64(int64* value)
 	return __atomic_load_n(value, __ATOMIC_ACQUIRE);
 }
 
+#ifdef __cplusplus
+}
+#endif
+
 
 #else	// __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)