AArch64: remove "arm64_be" support in favour of "aarch64_be".

There really is no arm64_be: it was a useful fiction to test big-endian support
while both backends existed in parallel, but now the only platform that uses
the name (iOS) doesn't have a big-endian variant, let alone one called
"arm64_be".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213748 91177308-0d34-0410-b5e6-96231b3b80d8