system: use getrandom() and fallback on urandom if syscall doesn't work

[illwieckz]

Sometime with less common compilers I get an error about getrandom syscall failures, but we already have an alternate implementation. It happens that the checks is currently done at build time, but then even if the headers are there, it may fail at run time. So we technically can try the syscall then the urandom read if the syscall fails.

What I don't know though, is that if doing that is reducing security or not.