I've found this thread helpful: https://kernelnewbies.kernelnewbies.narkive.com/rCAKczFM/regarding-getrandom-syscall#post10

There's a hint how to use the system call number (from some architecture docs I do not understand) to get around the missing syscalls.

Quoting here in case this gets lost:

Avantika Rawat
Hi All,

Thanks for the help

I was getting SYS_getrandom not defined compilation error as it is not defined in my c libraries. Also now i am able to call getrandom through syscall with proper syscall number which is arch dependent.
syscall (6314, buf, l , o);

I've done the same thing, luckily i was on the same arch as them, and it's worked.