Fix ability to override TARGET_GCC_VERSION in product makefiles

Change-Id: Id3ee409336740e81bb5459efd9ff57ee0bf6c08c
Add ARCH_VARIANT_CPU and TARGET_GCC_VERSION to dumpvar.mk

and add a printvars target, which will probably be useless, as it prints 90000000000000 variables.

Change-Id: I12786fb40374dc577b31b7f00e5338adf645e901