[PECOFF] Add a field for machine type to PECOFFLinkingContext.

So that we can determine what the target architecture is. Adding this
field does not mean that we are going to support non-i386 architectures
soon; there are many things to do to support them, and I'm focusing on
i386 now. But this is the first step toward multi architecture support.

llvm-svn: 190627