basic/util: add macro variants of log2 functions

basic/util: add macro variants of log2 functions

The macro variants can be used in static initializers.

The same guard against calling __builtin_clz(0) is added as for
__builtin_clzll(0), since that's undefined behaviour too. Our code
wouldn't call it, but this avoids a potential pitfall with the macro.
All variants map 0→0. Otherwise we'd often have to handle 0 specially
in callers.

__builtin_clz takes unsigned as the argument, so there's no LOG2I macro.