Hardcode qbe and harec to avoid the overhead of wrapping hare with a
bash script.
By doing that, we also had to change how `makeFlags` were defined: Now
only part of the `<ARCH>_<TOOL>` make variables are conditioned to the
enableCrossCompilation. The ones belonging to the build platform are
passed unconditionally.
Add qbe and harec to propagatedBuildInputs.
Some build frameworks may rely on having harec and qbe available on
PATH by setting `HAREC` and `QBE` to relative paths --- e. g., `haredo`.
