test: Fix explicit_bzero() test on the Hurd

test: Fix explicit_bzero() test on the Hurd

On the Hurd a small read(3) might end up (indirectly) copying the data
on the stack, which we will end up finding even when we have cleared
the buffer.

To avoid these side effects, we add a new function, that we force not
to be inlined, so that we can reuse the same stack space, that will
blank any possible stack side effects. This should be portable
regardless of stack growing up or down.

Diagnosis-by: Samuel Thibault <sthibault@debian.org>