Make `Bitness` undefined for the common variant

Bitness was introdcued in https://r.android.com/3283081 to make the
bitness of the jni lib dep explicit. However, this value is not well
defined when used on the common arch. To prevent inadvertent uses,
create a panic for the common arch.

Test: m nothing
Change-Id: I67724c4b874273b5a3c8f162d590c8a57a30b840