ARM: update comment to match reality

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197570 91177308-0d34-0410-b5e6-96231b3b80d8
One ppc32-darwin, a i64 inside a structure can have 32 bit alignment.

Thanks for Iain Sandoe for testing this with the original gcc.

Clang was already getting this right.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197572 91177308-0d34-0410-b5e6-96231b3b80d8