In r331279 the code used ENOSYS to check the existence of getrandom(2).
This will only work if the caller already handles SIGSYS, which is not
always the case.

Address this by checking osreldate instead. Note that because there
was not __FreeBSD_version bump when the system call was added, use
1200061 (r332100) which is the first bump after the introduction of
the system call.

PR: 230762
Reported by: Jenkins via Mark Millard
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D16807