[llvm-exegesis] Add an AArch64 target

The target does just enough to be able to run llvm-exegesis in latency mode for
at least some opcodes.

Differential Revision: https://reviews.llvm.org/D48780


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336187 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed