memcpy is leaking secret data through ZMM vector registers

Hi Jan,

Leaking register contents will always be true for all GLIBC functions that you might
call with "secrets" - that's unavoidable since clearing registers is neither the default,
nor something that assembly implementations support (and rebuilding everything
with -fzero-call-used-regs would add a huge amount of overhead...

The best solution is to not call library functions with "secrets" so you don't create that
problem. Alternatively, you could call memcpy with some non-secret data so that it
overwrites the vector registers. Quite easy as a solution without introducing new
symbols or having to rewrite assembly code to clear registers etc.

Cheers,
Wilco