Add get_architectures() and C++ versions

* get_architectures() returns the primary and the secondary
  architectures in one array. That turned out to be convenient.
* Add C++ versions for get[_secondary]_architectures(), returning a
  BStringList.

headers/private/system/architecture_private.h

@@ -16,6 +16,7 @@ const char*	__get_architecture();
 const char*	__get_primary_architecture();
 size_t		__get_secondary_architectures(const char** architectures,
 				size_t count);
+size_t		__get_architectures(const char** architectures, size_t count);
 const char*	__guess_architecture_for_path(const char* path);