Add TARGET_GCC_VERSION

Default to 4.6, but can overwritten to something else in
prebuilts/gcc/$os/$arch/$arch-linux-android*-$ver

Change-Id: I16f99d642b8ff8e831d392261a0c536041107051

Conflicts:
	core/config.mk
derp

Change-Id: I77c64dac5b747b792543a6632294f878ae76f093