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/combo/TARGET_linux-arm.mk
	core/combo/TARGET_linux-mips.mk