[PATCH] Generate SEH unwinding info on Win64

[Reid Kleckner]

I tried to commit today, but there were conflicts while rebasing.  I attempted to resolve them, but I still had this test failure after that:
  test/CodeGen/X86/x86-64-static-relo-movl.ll

Can you look into this?  The test uses a x86_64-pc-win32-macho triple, which makes us hit "LLVM ERROR: No open Win64 EH frame function!".  We probably shouldn't emit win64 style unwind info when generating MachO object files.

http://reviews.llvm.org/D4081