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

[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

llvm-svn: 341176