llvm/test/CodeGen/AArch64/cpus.ll: Tweak to use -mtriple=aarch64-unknown-unknown, or this would crash for targeting pecoff like *-mingw32.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201315 91177308-0d34-0410-b5e6-96231b3b80d8