Add a 's' specifications to AArch64.

This has no functionality change as clang adds explicit alignment info for
byval arguments. The only difference is that now the clang produced
DataLayout string for AArch64 is identical to the LLVM produced one.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197538 91177308-0d34-0410-b5e6-96231b3b80d8