mesa: Make bitset.h not rely on Mesa-specific types and functions.

mesa: Make bitset.h not rely on Mesa-specific types and functions.

Note that we can't use u_math.h's align() because it's a function instead
of a macro, while BITSET_DECLARE needs a constant expression for nouveau's
usage in global declarations.

v2: Stick some parens around the bits macro argument usage (review by Jose).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>