Improve randomness syscalls

[newpavlov]

The RDRAND code used in generate_random_number32/64 does not contain a workaround for the AMD glitch. You can take a look at the getrandom code for reference.

Also it would be nice if instead of generate_random_number32/64 you will expose a Linux-like interface, i.e. a syscall which will work on buffers.

[stlankes]

Thanks for the suggestions! I will work on it.