Add an MCSubtargetInfo variable to the TargetMachine.

Add an MCSubtargetInfo variable to the TargetMachine.

This enables us to remove calls to the subtarget from the TargetMachine
and with a small hack for backends that require global subtarget
information for module level code generation, e.g. mips abi flags, as
mentioned in a fixme in the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232776 91177308-0d34-0410-b5e6-96231b3b80d8