Driver: only inject libstdc++ paths when needed on CrossWindows

When building with libc++ don't bother with injecting the libstdc++
search paths into the linker search path.  This will make it easier to
switch between ld and lld.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302184 91177308-0d34-0410-b5e6-96231b3b80d8