Move to a private function to initialize subtarget dependencies

Move to a private function to initialize subtarget dependencies
so we can use initializer lists for the ARMSubtarget and then
use this to initialize a moved DataLayout on the subtarget from
the TargetMachine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210861 91177308-0d34-0410-b5e6-96231b3b80d8