util/rand_xor: use getrandom() when available

util/rand_xor: use getrandom() when available

This function has been added in glibc 2.25, and the related syscall in
Linux 3.17, in order to avoid requiring the /dev/urandom to exist, and
doing the open()/read()/close() dance on it.

We pass GRND_NONBLOCK so that it doesn’t block if not enough entropy has
been gathered to initialise the /dev/urandom source, and fallback to the
next source in any error case.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2026>