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.

llvm-svn: 178870