Add benchmark for getresuid(2) - three pointers, although only "output" ones,
and quite lightwait.

The purpose of this commit, and the previous one, is to be able to measure
overhead of pointer arguments - in case you're running a strange architecture
where pointers and integers are quite different things at the hardware level.

Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA, AFRL