Regularize use of PRIVATE_CXX.

Change-Id: Ifc628c67b5fa142cec59187f89f2d39df81160b9
diff --git a/core/combo/HOST_darwin-x86.mk b/core/combo/HOST_darwin-x86.mk
index a136041..855317f 100644
--- a/core/combo/HOST_darwin-x86.mk
+++ b/core/combo/HOST_darwin-x86.mk
@@ -59,7 +59,7 @@
 HOST_CUSTOM_LD_COMMAND := true
 
 define transform-host-o-to-shared-lib-inner
-    $(HOST_CXX) \
+    $(PRIVATE_CXX) \
         -dynamiclib -single_module -read_only_relocs suppress \
         $(HOST_GLOBAL_LD_DIRS) \
         $(HOST_GLOBAL_LDFLAGS) \
@@ -74,7 +74,7 @@
 endef
 
 define transform-host-o-to-executable-inner
-$(HOST_CXX) \
+$(PRIVATE_CXX) \
         -o $@ \
         -Wl,-dynamic -headerpad_max_install_names \
         $(HOST_GLOBAL_LD_DIRS) \