Add a non-const subtarget returning function to the target machine

Add a non-const subtarget returning function to the target machine
so that we can use it to get the old-style JIT out of the subtarget.

This code should be removed when the old-style JIT is removed
(imminently).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214560 91177308-0d34-0410-b5e6-96231b3b80d8