ghcHEAD: bump to 8.11.20200403 (#84217)

* ghcHEAD: bump to 8.11.20200403

* ghcHead: reduce diff vs. 8.10.1

dontAddExtraLibs was removed by accident (IMO) in ea19a8ed

* ghcHEAD: add ability to use system libffi

- enable nixpkgs' libffi
- minimise diffs against 8.10.1
- remove patching

* remove configure warning about --with-curses-includes

configure: WARNING: unrecognized options: --with-curses-includes