Regularize use of PRIVATE_CXX.

Change-Id: Ifc628c67b5fa142cec59187f89f2d39df81160b9
diff --git a/core/combo/TARGET_linux-arm.mk b/core/combo/TARGET_linux-arm.mk
index 506de1b..ebb6590 100644
--- a/core/combo/TARGET_linux-arm.mk
+++ b/core/combo/TARGET_linux-arm.mk
@@ -231,7 +231,7 @@
 endif
 
 define transform-o-to-shared-lib-inner
-$(TARGET_CXX) \
+$(PRIVATE_CXX) \
 	-nostdlib -Wl,-soname,$(notdir $@) -Wl,-T,$(BUILD_SYSTEM)/armelf.xsc \
 	-Wl,--gc-sections \
 	-Wl,-shared,-Bsymbolic \
@@ -252,7 +252,7 @@
 endef
 
 define transform-o-to-executable-inner
-$(TARGET_CXX) -nostdlib -Bdynamic -Wl,-T,$(BUILD_SYSTEM)/armelf.x \
+$(PRIVATE_CXX) -nostdlib -Bdynamic -Wl,-T,$(BUILD_SYSTEM)/armelf.x \
 	-Wl,-dynamic-linker,/system/bin/linker \
     -Wl,--gc-sections \
 	-Wl,-z,nocopyreloc \
@@ -271,7 +271,7 @@
 endef
 
 define transform-o-to-static-executable-inner
-$(TARGET_CXX) -nostdlib -Bstatic -Wl,-T,$(BUILD_SYSTEM)/armelf.x \
+$(PRIVATE_CXX) -nostdlib -Bstatic -Wl,-T,$(BUILD_SYSTEM)/armelf.x \
     -Wl,--gc-sections \
 	-o $@ \
 	$(TARGET_GLOBAL_LD_DIRS) \