runtime: correct attribution of internal mutex contention about go

[prattmic]

At a glance, this plan seems reasonable to me.

One note on assumed constraints:

We cannot change the size of a runtime.mutex, it needs to remain uintptr.

While it is nice to avoid increasing the size of a mutex, I don't think this needs to be a hard requirement. Taking a look at where we use mutex, it looks like timers and channels probably have the most objects and would thus have the biggest impact. But neither of those structures is tiny, so I don't think it would be the end of the world if they get bigger.