Make llvm useable on Windows

By default llvm uses copies of the binaries to provide the different flavors.

Those copies can be huge, 100mb for clang.

Craft facilitates kshim instead.

To make that work however we need to remove a hack from llvm which replaces Argv0 with the current binary file name.

The argv0 behavior was introduced in https://github.com/llvm-mirror/llvm/commit/7c4ee81607aa8cbd45e63460c8c9224bdc06a897