Use CLOCK_REALTIME for providing the initial seed value

This is equivalent to the changes in checkasm!51 (merged), and covers the seek_stress test as well.

CLOCK_MONOTONIC is specified as returning time "since an unspecified point in the past". On RISC OS with UnixLib this returns the time since the last hard reset, but with SharedCLibrary this returns the time since the program started - combined with the coarse resolution used internally, this almost always results in a seed of 0.

CLOCK_REALTIME meanwhile is specified as returning time since the epoch, so it should behave consistently across all platforms.