[llvm-exegesis] Add pfm counters for Zen2 (znver2).

Summary: There are no counters for individual ports, but this is already
enough to find a lot of issues in the current model (upcoming patch).

Reviewers: dblaikie, gchatelet

Subscribers: hiraditya, tschuett, RKSimon, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72032