Panic messages no longer include current platform

Since one of the recent MRs by @Ericson2314 the panic message no longer contains information about the current target.

I tried to add back the platform to the message but once !555 (merged) lands then the module which defines panic (PlainPanic) no longer can depend on DynFlags so we're stuck.

This ticket is to ask the question about how much we care about the user's target appearing in a panic. Personally I think it's often useful when a user reports a bug without much information.