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/envsetup.sh b/envsetup.sh
index a88d8ce..2d9d245 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -124,11 +124,11 @@
     export ANDROID_EABI_TOOLCHAIN=
     local ARCH=$(get_build_var TARGET_ARCH)
     case $ARCH in
-        x86) toolchaindir=x86/i686-linux-android-4.6/bin
+        x86) toolchaindir=x86/i686-linux-android-$targetgccversion/bin
             ;;
         arm) toolchaindir=arm/arm-linux-androideabi-$targetgccversion/bin
             ;;
-        mips) toolchaindir=mips/mipsel-linux-android-4.6/bin
+        mips) toolchaindir=mips/mipsel-linux-android-$targetgccversion/bin
             ;;
         *)
             echo "Can't find toolchain for unknown architecture: $ARCH"