Hadrian: Use the Cabal build script on Windows by default

The Stack build script `build.stack.bat` currently fails on Windows
when invoked with the `--configure` flag, see:

https://ghc.haskell.org/trac/ghc/ticket/15982

The Cabal build script `build.cabal.bat` works reliably on my Windows
machine, so I am proposing to switch to it by default, that is, to run
it from the default `build.bat` script.

The Stack build script can still be run directly if need be.