[MinGW] [ARM] Add stubs for potential automatic dllimported variables

The runtime pseudo relocations can't handle the ARM format embedded
addresses in movw/movt pairs. By using stubs, the potentially
dllimported addresses can be touched up by the runtime pseudo relocation
framework.

Differential Revision: https://reviews.llvm.org/D51450

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341176 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed