util-random: workaround getrandom unavailability

util-random: workaround getrandom unavailability

getrandom syscall availability is detected at runtime. So it is
possible that the build is done on a box that supports it but
the run is done on a system with no availability. So a workaround
solution is needed to fix this case.

Also we have seen some issue in docker environment where the build
is detecting getrandom but where it does not work at runtime.

For both reasons, the code is updated to have a call to a fallback
function if ever the getrandom call returns that the syscall is
not available.