Add AArch64 big endian Target (aarch64_be)

[scott douglass]

> This patch adds a big endian AArch64 target description.
> (A corresponding target machine has been committed to llvm-commits.)
> Please review.

Thanks, it looks sensible to me.  I have only some superficial style
comments:

+    llvm::Triple::ArchType arch =
getASTContext().getTargetInfo().getTriple().getArch();

Please keep lines to 80 columns; local variables should be initial upper
case ('Arch').

+    setDescriptionString ();

No space before the '('.

(See also http://llvm.org/docs/CodingStandards.html)