Replace occurence of nested comments in blake2 reference implementation
with preprocessor directive for disabling unused code.
`blake2s-load-xop.h` is conditionally pulled in only on chips with XOP
support, among others the AMD Bulldozer. The malformed comments in the
source file breaks the build of `hashlib`'s `_blake2` on GCC 6.3.0.
Official reference code on github uses `#if` so this change should be
uncontroversial.
David Carlier
Lars Viklund
Christian Heimes