Clean up the x86-64 bigint_mul_add_words implementation a bit using the new register macros.
Clean up the x86-64 bigint_mul_add_words implementation a bit using the
new register macros.

Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan