[PATCH] Fix the prefix for arm64 triple

[Yi Kong]

>>! In D4516#8, @t.p.northover wrote:
> It looks like it's only used by that part of Clang. We probably want a test though (even if it's in Clang rather than LLVM), since this is clearly causing problems for someone.

I have written a test case in Clang, however due to the limitation of Phabricator, I can't post it in the same post. I will take another look to see whether it is possible to test in LLVM.
 
> Also, are you planning to do the more thorough triple merge, Yi?

Although the merge should be trivial, but I think it would be best for you guys to do it to avoid breaking your internal Apple clients.

http://reviews.llvm.org/D4516