shared: Let bit asserts take a bit instead of a position

It's much more common to have bits defined as enums than having a
position, so let's just directly take a bit as argument to the bit
asserts. For the few cases where only the position is available, it's
easy to get the right bit using a shift.

The is_pow2_64() helper functions have been added in order to validate
the bit passed as argument and prevent programming errors.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>