[Driver] Create "link[.exe]" symlink for the Windows driver.

Clang creates "clang-cl" as a symlink to (or a copy of) "clang" for the MSVC-
compatible driver. This patch is to do the same thing for "link" and "lld".

Differential Revision: http://llvm-reviews.chandlerc.com/D3066
[PECOFF] Handle objects with unknown machine type header value.

An object whose machine type header value is unknown looks a bit odd but
is valid. If an object contains only machine-type-independent data, you
can leave the type field unspecified. Some files in oldname.lib are such
object files.