Do not link ghc stage1 using -threaded, only for stage2 or 3

We link stage1 using the bootstrapping compiler and there's
no guarantee that it has working support for threaded

ghc/ghc.mk

@@ -29,7 +29,8 @@ endif
 
 ifeq "$(GhcThreaded)" "YES"
 # Use threaded RTS with GHCi, so threads don't get blocked at the prompt.
-ghc_HC_OPTS += -threaded
+ghc_stage2_HC_OPTS += -threaded
+ghc_stage3_HC_OPTS += -threaded
 endif
 
 ifeq "$(BuildSharedLibs)" "YES"