AArch64: strengthen Darwin ABI alignment assumptions

A global variable without an explicit alignment specified should be assumed to
be ABI-aligned according to its type, like on other platforms. This allows us
to use better memory operations when accessing it.

rdar://18533701

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223180 91177308-0d34-0410-b5e6-96231b3b80d8