AArch64: use aarch64_be instead of arm64_be in all tests.

arm64_be doesn't really exist; it was useful for testing while AArch64 and
ARM64 were separate, but now the only real way to refer to the system is
aarch64_be.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213747 91177308-0d34-0410-b5e6-96231b3b80d8