Add a 'SINGLETON' directive to kobj interface definition

[jhibbits]

Summary

This makes the interface described in the definition file act like a
pseudo-IFUNC service, by caching the found method locally.

Applying this to the PowerPC MMU definitions, it yields a significant
(15-20%) performance improvement, seen in both a 'make buildworld' and a
parallel build of LLVM.

[jhb]

I would perhaps be inclined to just not use kobj for things that are performance sensitive to be honest. You aren't going to have kldload'able pmaps, so the ABI "win" that kobj gives you isn't really relevant. A simple struct with function pointers would probably give you even lower overhead than the singleton approach.

[imp]

In D23245#509650, @jhb wrote:

I would perhaps be inclined to just not use kobj for things that are performance sensitive to be honest. You aren't going to have kldload'able pmaps, so the ABI "win" that kobj gives you isn't really relevant. A simple struct with function pointers would probably give you even lower overhead than the singleton approach.

I suspect that's another 5-7% as well. I click ok because it was an easy win and made this point to somebody 10 or 15 years ago at BSDcan.