Check for __SIZEOF_INT128__ before using TI mode
Check for __SIZEOF_INT128__ before using TI mode

Otherwise we run into problems on 64-bit CPUs with 32-bit userland.
GH #563

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