distclean was removing ghc-pwd/dist, should be ghc-pwd/dist-boot

This has been causing bloat in the src dist for ages.

Noticed while looking at #6009, but I don't think this is the bug
(./configure always removes ghc-pwd/dist-boot before building ghc-pwd)

ghc.mk

@@ -1234,7 +1234,7 @@ distclean : clean
 	$(call removeFiles,libraries/process/include/HsProcessConfig.h)
 	$(call removeFiles,libraries/unix/include/HsUnixConfig.h)
 	$(call removeFiles,libraries/old-time/include/HsTimeConfig.h)
-	$(call removeTrees,utils/ghc-pwd/dist)
+	$(call removeTrees,utils/ghc-pwd/dist-boot)
 	$(call removeTrees,inplace)
 	$(call removeTrees,$(patsubst %, libraries/%/autom4te.cache, $(PACKAGES_STAGE1) $(PACKAGES_STAGE2)))