Fix HC porting test in makefiles

Now that we are trying to support cross compilation, we can't use
    "$(TARGETPLATFORM)" != "$(HOSTPLATFORM)"
as a test for HC-porting.

includes/ghc.mk

@@ -46,7 +46,7 @@ endif
 
 ifneq "$(BINDIST)" "YES"
 
-ifneq "$(TARGETPLATFORM)"  "$(HOSTPLATFORM)"
+ifeq "$(PORTING_HOST)" "YES"
 
 $(includes_H_CONFIG) :
 	@echo "*** Cross-compiling: please copy $(includes_H_CONFIG) from the target system"
@@ -116,7 +116,7 @@ endif
 
 includes_DERIVEDCONSTANTS = includes/DerivedConstants.h
 
-ifneq "$(TARGETPLATFORM)" "$(HOSTPLATFORM)"
+ifeq "$(PORTING_HOST)" "YES"
 
 DerivedConstants.h :
 	@echo "*** Cross-compiling: please copy DerivedConstants.h from the target system"
@@ -144,7 +144,7 @@ endif
 
 includes_GHCCONSTANTS = includes/GHCConstants.h
 
-ifneq "$(TARGETPLATFORM)" "$(HOSTPLATFORM)"
+ifeq "$(PORTING_HOST)" "YES"
 
 $(includes_GHCCONSTANTS) :
 	@echo "*** Cross-compiling: please copy DerivedConstants.h from the target system"

rts/ghc.mk

@@ -44,7 +44,7 @@ rts_C_SRCS = $(filter-out $(EXCLUDED_SRCS),$(wildcard rts/*.c $(foreach dir,$(AL
 rts_CMM_SRCS = $(wildcard rts/*.cmm)
 
 # Don't compile .S files when bootstrapping a new arch
-ifeq "$(TARGETPLATFORM)" "$(HOSTPLATFORM)"
+ifneq "$(PORTING_HOST)" "YES"
 ifneq "$(findstring $(TargetArch_CPP), powerpc powerpc64)" ""
 rts_S_SRCS += rts/AdjustorAsm.S
 else