Add `-haddock` option under ci condition to fix #16415

In order to use the `:doc` command in ghci, it is necessary
to compile for core libraries with `-haddock` option.

Especially, the `-haddock` option is essential for release building.

Note:
  * The `-haddock` option may affect compile time and binary size.
  * But hadrian has already set `-haddock` as the default.
  * This patch affects the make-based building.

This patch has been split from !532.

.circleci/prepare-system.sh

@@ -29,6 +29,7 @@ cat <<EOF >> mk/build.mk
 BuildFlavour=$BUILD_FLAVOUR
 ifneq "\$(BuildFlavour)" ""
 include mk/flavours/\$(BuildFlavour).mk
+GhcLibHcOpts+=-haddock
 endif
 EOF