MC: create X86WinCOFFStreamer for target specific behaviour

This introduces a target specific streamer, X86WinCOFFStreamer, which handles
the target specific behaviour (e.g. WinEH).  This is mostly to ensure that
differences between ARM and X86 remain disjoint and do not accidentally cross
boundaries.  This is the final staging change for enabling object emission for
Windows on ARM.
Add WoA object file emission support

Introduce support for WoA PE/COFF object file emission from LLVM.  Add the new
target specific PE/COFF Streamer (ARMWinCOFFStreamer) that handles the ARM
specific behaviour of PE/COFF object emission.  ARM exception information is not
yet emitted and is a TODO item.

The ARM specific object writer (ARMWinCOFFObjectWriter) handles the ARM specific
relocation handling in conjunction with the WinCOFFObjectWriter in the MC layer.
The MC layer needs to be updated to deal with the relocation adjustments.
Branch relocations are adjusted by 4 bytes (unlikely their ELF counterparts).

Minor tweaks to switch multiple conditional checks into equivalent switch
statements.  The ObjectFileInfo is updated to relax the object file setup for
Windows COFF.  Move the architecture checks into an assertion.  Windows COFF is
currently only supported on x86, x86_64, and ARM (thumb).  Rather than
defaulting to ELF, we will refuse to generate an object file.  This is better
though as you do not get an (arbitrary) object file which is different from the
request.