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-x86.mk b/core/combo/TARGET_linux-x86.mk
index 948ee17..187e2c2 100644
--- a/core/combo/TARGET_linux-x86.mk
+++ b/core/combo/TARGET_linux-x86.mk
@@ -22,6 +22,10 @@
 TARGET_ARCH_VARIANT := x86
 endif
 
+ifeq ($(strip $(TARGET_GCC_VERSION)),)
+TARGET_GCC_VERSION := 4.6
+endif
+
 # Include the arch-variant-specific configuration file.
 # Its role is to define various ARCH_X86_HAVE_XXX feature macros,
 # plus initial values for TARGET_GLOBAL_CFLAGS