[PATCH] Fix the prefix for arm64 triple

[Tim Northover]

> 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.

No need to do that. I suspect it's very strongly linked to the C++
front-end so I'd be quite happy with a Clang test, but we should have
one.

Cheers.

Tim.