[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) Scheduler descriptions are yet to be put in place.

Reviewers: craig.topper

Differential Revision: https://reviews.llvm.org/D58343

llvm-svn: 354897