[X86] AMD znver2 enablement

This patch enables the following

1) AMD family 17h "znver2" tune flag (-march, -mcpu).
2) ISAs that are enabled for "znver2" architecture.
3) For the time being, it uses the znver1 scheduler model.
4) Tests are updated.
5) This patch is the clang counterpart to D58343

Reviewers: craig.topper
Tags: #clang

Differential Revision: https://reviews.llvm.org/D58344

llvm-svn: 354899