Use PRIVATE_CXX in HOST modules.

This fixes the build error:
out/host/linux-x86/obj/SHARED_LIBRARIES/libneo_util_intermediates/neo_err.o:
file not recognized: File format not recognized
See also CL #84578

Change-Id: Ib91230c7a5c9809eb1935959eaa9be8fa7c596b9

core/definitions.mk

@@ -1001,7 +1001,7 @@ endef
 # it to be overriden en-masse see combo/linux-arm.make for an example.
 ifneq ($(HOST_CUSTOM_LD_COMMAND),true)
 define transform-host-o-to-shared-lib-inner
-$(HOST_CXX) \
+$(PRIVATE_CXX) \
 	-Wl,-rpath-link=$(TARGET_OUT_INTERMEDIATE_LIBRARIES) \
 	-Wl,-rpath,\$$ORIGIN/../lib \
 	-shared -Wl,-soname,$(notdir $@) \
@@ -1164,7 +1164,7 @@ endef
 
 ifneq ($(HOST_CUSTOM_LD_COMMAND),true)
 define transform-host-o-to-executable-inner
-$(HOST_CXX) \
+$(PRIVATE_CXX) \
 	-Wl,-rpath-link=$(TARGET_OUT_INTERMEDIATE_LIBRARIES) \
 	-Wl,-rpath,\$$ORIGIN/../lib \
 	$(HOST_GLOBAL_LD_DIRS) \