Add a way to divert certain headers which are ISA specific.
Add a way to divert certain headers which are ISA specific.

Otherwise the amalgamation internal header ended up with AVX2 code
in it, which caused problems for the non-AVX2 enabled files.

webmaster128/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan,webmaster128/botan,randombit/botan,webmaster128/botan,randombit/botan,randombit/botan