ARM: improve WoA ABI conformance for frame register

Windows on ARM uses R11 for the frame pointer even though the environment is a
pure Thumb-2, thumb-only environment.  Replicate this behaviour to improve
Windows ABI compatibility.  This register is used for fast stack walking, and
thus is part of the Windows ABI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209085 91177308-0d34-0410-b5e6-96231b3b80d8