objdump: identify WoA WinCOFF/ARM correctly

Since LLVM currently only supports WinCOFF, assume that the input is WinCOFF
rather than another type of COFF file (ECOFF/XCOFF).  If the architecture is
detected as thumb (e.g. the file has a IMAGE_FILE_MACHINE_ARMNT magic) then use
a triple of thumbv7-windows.

This allows for objdump to properly handle WoA object files without having to
specify the target triple manually.

llvm-svn: 206446