[ARM64] Add a big endian version of the ARM64 target machine, and update all users.

This completes the porting of r202024 (cpirker "Add AArch64 big endian Target (aarch64_be)") to ARM64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206965 91177308-0d34-0410-b5e6-96231b3b80d8