Add `-haddock` to perf.mk rather than prepare-system.sh

To cover ci conditions from ghc8.6 to 8.9, I add `-haddock` option
to `mk/flavours/perf.mk` rather than `.circleci/prepare-system.sh`.

Because in windows condition of ghc-8.9, `mk/flavours/*` is included
after `prepare-system.sh`.

In addition, in linux condition of ghc-8.6, `mk/flavors/perf.mk` is used.

.circleci/prepare-system.sh

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

mk/flavours/perf.mk

 SRC_HC_OPTS        = -O -H64m
 GhcStage1HcOpts    = -O2
 GhcStage2HcOpts    = -O2
-GhcLibHcOpts       = -O2
+GhcLibHcOpts       = -O2 -haddock
 BUILD_PROF_LIBS    = YES
 #HADDOCK_DOCS
 #BUILD_SPHINX_HTML