Add a 'SINGLETON' directive to kobj interface definition

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, on a POWER9 system.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D23245