Add ossl_num_bits(), which returns the significant number of bits in a size_t

Add ossl_num_bits(), which returns the significant number of bits in a size_t

This existed as an isolated static function in crypto/asn1/x_long.c, but
is really a pretty generic integer function, so it will serve better by
being exactly that.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29203)