Enable s2n-bignum word_clz() on amd64.

Enable s2n-bignum word_clz() on amd64.

The BN_num_bits_word() function is a hot path, being called more than
80 million times during a libcrypto regress run. The word_clz()
implementation uses five instructions to do the same as the generic code
that uses more than 60 instructions.

Discussed with tb@