We added support for the GNUstep ObjC runtime in 0b6264738f3d. In order to check if the target process uses
GNUstep we run an expensive symbol lookup in CreateInstance(). This turned out to cause a heavy performance
regression for non-GNUstep inferiors.
This patch puts a cheaper check in front, so that the vast majority of requests should return early. This
should fix the symptom for the moment. The conceptual question is why LanguageRuntime::FindPlugin invokes
create_callback for each available runtime unconditionally in every Process::ModulesDidLoad.