haskell: explicitly import Prelude hiding Word

everywhere where it can clash with Word type defined by SEL4.

riscv haskell: update arch thread and interrupt

riscv haskell: minimum compiling skeleton

All functions and datatypes required by generic code are supplied in
stubbed out or placeholder form, and anything not required is missing.

The code compiles (make build-riscv), please let's keep it that way.

Look for the "FIXME RISCV" tag, which is the official FIXME tag for this
project.