[Toybox] getrandom(2)

[enh]

i'm not sure getrandom(2) is a good choice for a compile-time probe.
certainly on Android it's common for your libc to be way ahead of your
kernel. in this specific case, on Android getrandom(2) is very likely
to return ENOSYS. for the arc4random functions (from BSD) we'll try
getrandom but fall back to open/read/close. seems like xgetrandom
might be better off doing that, on the assumption that folks will try
to use prebuilts on old devices/hosts?