Fix the prefix for arm64 triple

Triple.cpp still returns "arm64" as prefix for arm64 triple, causing Clang not
being able to select the correct GCCBuiltin IR.

This patch changes the value to correct prefix "aarch64". Regression test will
be added in the coming patch.

Differential Revision: http://reviews.llvm.org/D4516


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213240 91177308-0d34-0410-b5e6-96231b3b80d8