Define TARGET_GCC_VERSION in target specific make files.

So that GCC refresh can be performed in a target specific way.

Change-Id: I867e8b8fd671bef8c5af300e657a6da4fface032
diff --git a/core/combo/TARGET_linux-mips.mk b/core/combo/TARGET_linux-mips.mk
index 6e9cd01..dc28018 100644
--- a/core/combo/TARGET_linux-mips.mk
+++ b/core/combo/TARGET_linux-mips.mk
@@ -34,6 +34,10 @@
 TARGET_ARCH_VARIANT := mips32r2-fp
 endif
 
+ifeq ($(strip $(TARGET_GCC_VERSION)),)
+TARGET_GCC_VERSION := 4.6
+endif
+
 TARGET_ARCH_SPECIFIC_MAKEFILE := $(BUILD_COMBOS)/arch/$(TARGET_ARCH)/$(TARGET_ARCH_VARIANT).mk
 ifeq ($(strip $(wildcard $(TARGET_ARCH_SPECIFIC_MAKEFILE))),)
 $(error Unknown MIPS architecture variant: $(TARGET_ARCH_VARIANT))