[ARM] Extend the usage of TARGET_GCC_VERSION for gdb and arm-eabi-*.

In the future to upgrade the gcc version we just need to edit one
line in core/config.mk.

Change-Id: I01bb38c38f2388913bb7369aa4b2da480411dfbe
Export TARGET_GCC_VERSION as an env variable.

Have it referenced by the adbs tool to pick the specified toolchain.

Change-Id: I23df88946254a55d0749d6753e54559faf33e121