expose TARGET_GCC_VERSION to dumpvar

This will only be exposed doing lunch if
TARGET_GCC_VERSION_EXP is set for the selected
device. TARGET_GCC_VERSION_EXP can be :
4.6 or 4.7 or 4.8 or 4.8-linaro or 4.9-linaro
4.7 is default

Change-Id: I0ea4df63316b80af4c15aade2753fa6f08fa1243