Build xhtml and haddock only when `HADDOCK_DOCS=YES`

This fixes #8558

ghc.mk

@@ -425,7 +425,10 @@ PACKAGES_STAGE2 += haskell98
 PACKAGES_STAGE2 += haskell2010
 endif
 
+ifeq "$(HADDOCK_DOCS)" "YES"
 PACKAGES_STAGE1 += xhtml
+endif
+
 ifeq "$(Windows_Target)" "NO"
 ifneq "$(TargetOS_CPP)" "ios"
 PACKAGES_STAGE1 += terminfo
@@ -665,8 +668,11 @@ else ifneq "$(findstring clean,$(MAKECMDGOALS))" ""
 BUILD_DIRS += libraries/integer-gmp2/gmp
 endif
 
+ifeq "$(HADDOCK_DOCS)" "YES"
 BUILD_DIRS += utils/haddock
 BUILD_DIRS += utils/haddock/doc
+endif
+
 BUILD_DIRS += compiler
 BUILD_DIRS += utils/hsc2hs
 BUILD_DIRS += utils/ghc-pkg