Add GCConfig::hs_err_name() to avoid GC-specific code in error reporting

The gc_mode() function in vmErrror.cpp currently tests which Use*GC flag is set and translates that into a name. We should instead call into GCConfig to get the name. This is not placed on CollectedHeap, since we might need this name before a CollectedHeap instance has been created, e.g. if we crash during VM startup.