[gnustep-objc] Make the GNUstep v2 ABI work for Windows DLLs.

Summary:
Based on a patch by Dustin Howett, modified to not change the ABI for
ELF platforms.

Use more Windows-like section names.

This also makes things more readable by PE/COFF debug tools that assume
sections fit in the first header.

With these changes in, it is now possible to build a working WinObjC
with clang and the WinObjC version of GNUstep libobjc (upstream GNUstep
libobjc + a work around for incremental linking, which can be removed
once LINK.EXE gains a feature to opt sections out of receiving extra
padding during an incremental link).

Patch by Dustin Howett!

Reviewers: DHowett-MSFT

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58724

llvm-svn: 357364