rts: Disable --eventlog-flush-interval unless compiled with -threaded.
This MR fixes issue #26222 (closed): Using --eventlog-flush-interval with the non-threaded RTS leads to eventlog corruption.
This MR makes three changes when code is compiled against the non-threaded RTS:
- It disables the --eventlog-flush-interval flag.
- It disables the documentation for the --eventlog-flush-interval flag.
- It disables the relevant state from RtsConfig and code from Timer.