ARM: support PIC on Windows on ARM

Handle lowering of global addresses for PIC mode compilation on Windows.  Always
use the movw/movt load to load the address as Windows on ARM requires ARMv7+ and
is a pure Thumb environment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208385 91177308-0d34-0410-b5e6-96231b3b80d8