Windows support in libobjc2

[David Chisnall]

Hello,

I have spent a little bit of time integrating some of the work done by the 
WinObjC team with libobjc2 and now have the relevant clang changes under review:

https://reviews.llvm.org/D50144

With this clang patch, it is now possible to build an objc.dll that supports 
the new ABI (with all of the introspection improvements and reduced binary size 
that this includes) as well as providing SEH-compatible exception support (so 
you can throw exceptions through MSVC-compiled C/C++ code and everything 
works).  I’ve tested with the runtime’s test suite on i386 and x64 Windows.  
Incremental linking doesn’t work, but everything else does.  I have not tested 
with linkers other than Microsoft’s LINK.EXE, but compatible linkers ought to 
work.

Building on Windows is still a little bit painful, but I can provide binaries 
of clang and the runtime if anyone wants to test a bit more.  Is anyone using 
Windows and GNUstep?

David