Fix HaikuBuildCompatibility to not define atmics anymore.

This reverts commit 34dbbb65fd.
Instead, we can remove thos from HaikuBuildCompatibility and things will work fine, unless one try t build Haiku on BeOS (this isn't
supported anymore) or a very old Haiku which esdon't have those.

headers/os/support/SupportDefs.h

@@ -217,9 +217,6 @@ 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)
@@ -322,10 +319,6 @@ atomic_get64(int64* value)
 	return __atomic_load_n(value, __ATOMIC_ACQUIRE);
 }
 
-#ifdef __cplusplus
-}
-#endif
-
 
 #else	// __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)