[AArch64] Remove .data_region directive from AArch64.

.data_region is only used in Darwin, so it shouldn't be generated
for other OS. Currently AArch64 doesn't support darwin yet, so
I removed it from AArch64. When Darwin is supported someday, we can
add it back and associate it with Darwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204424 91177308-0d34-0410-b5e6-96231b3b80d8