[PATCH] Fix the prefix for arm64 triple

[Yi Kong]

>>! In D4516#11, @t.p.northover wrote:
> 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.

My plan to implement this is by D4521, where GCCBuiltin __builtin_arm_[dmb, dsb, isb] are tested to be correctly selected by Clang when arm64 triple is supplied. It's an indirect test, but that's the only test I can think of.

Do you have any suggestion? Otherwise ok to commit?

http://reviews.llvm.org/D4516