AArch64: bring predefines in line with most recent ACLE document

The prefixes and names used are now identical to 32-bit ARM, which is also
expected to remain unchanged.

If we made this change after a release, we'd probably have to support both
variants for a while, but I think since AArch64 exists only on trunk now, it's
acceptable to simply swap them now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178870 91177308-0d34-0410-b5e6-96231b3b80d8