Reserve option to use BN_mod_exp_mont_consttime in ECDSA.

Submitted by Shay Gueron, Intel Corp.
RT: 3149

Reviewed-by: Rich Salz <rsalz@openssl.org>
Add ECP_NISTZ256 by Shay Gueron, Intel Corp.

RT: 3149

Reviewed-by: Rich Salz <rsalz@openssl.org>
Configure: engage ECP_NISTZ256.

RT: 3149

Reviewed-by: Rich Salz <rsalz@openssl.org>
crypto/bn/asm/x86_64-mont*.pl: add missing clang detection.

Reviewed-by: Rich Salz <rsalz@openssl.org>