Driver: add a cygwin linker tool

This adds a linker tool for the Windows cygwin environment.  This linker
invocation is significantly different from the generic ld invocation.  It
requires additional parameters as well as does not accept some normal
parameters.  This should fix self-hosting on Cygwin.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211995 91177308-0d34-0410-b5e6-96231b3b80d8