Hadrian: always generate the libffi dynlibs manifest with globbing

Instead of trying to deduce which dynlibs are expected to be found (and then copied to the RTS's build dir) in libffi's build directory, with some OS specific logic, we now always just use getDirectoryFilesIO to look for those dynlibs and record their names in the manifest. The previous logic ended up causing problems on Windows, where we don't build dynlibs at all for now but the manifest file's logic didn't take that into account because it was only partially reproducing the criterions that determine whether or not we will be building shared libraries.

This patch also re-enables the Hadrian/Windows CI job, which was recently failing to build GHC precisely because of libffi shared libraries and the aforementionned duplicated logic.

@shayne-fletcher-da This patch should address the problem you reported on ghc-devs@.