Hadrian: do not cache GHC configure rule and Libffi rule now `produces` dynamic library files.
We should not cache the configure step as it doesn't track inputs accurately enough. E.g. If you change the ghc source directory then configure should generate a different output but the rule will fail to rerun.
The libffi build rule did not declare it's outputs. This fixes a missing
libffi.so error when doing a cloud build.