[llvm-dlltool] Set a proper machine type for weak symbol object files

This makes GNU binutils not reject the libraries outright.

GNU ld handles weak externals slightly differently though, so it
can't use them for aliases in import libraries, but this makes GNU
ld able to use the rest of the import libraries.

LLD accepted object files with machine type 0 aka
IMAGE_FILE_MACHINE_UNKNOWN.

Differential Revision: https://reviews.llvm.org/D59742

llvm-svn: 356982