MC: rework static_assert to be MSVC compatible

Visual Studio does not permit referencing a structure member as a static field
for sizeof calculations.  Resort to a pointer cast which is compatible across
Visual Studio and other compilers.
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.