Provide and use bn_clzw() in place of bn_word_clz().

Provide and use bn_clzw() in place of bn_word_clz().

On some architectures, we can provide an optimised (often single
instruction) count-leading-zero implementation. In order to do this
effectively, provide bn_clzw() as a static inline that can be replaced
by an architecture specific version. The default implementation defers
to the bn_word_clz() function (which may also be architecture specific).

ok tb@