Driver: Fix include directories when not using libgcc under mingw

Summary:
When we want to use mingw-w64 and clang with compiler-rt we should not
need to have libgcc installed. This fixes finding includes when libgcc
is not installed

Reviewers: yaron.keren

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11808

llvm-svn: 244902