evl/thread: enable notification on schedule overrun (T_WOSO)

Some scheduler policies may want to notify a thread which overruns its
allotted runtime. Introduce a new HM event - namely EVL_HMDIAG_OVERRUN
- which can be received via the common HM delivery channels (signal or
observable).

Notification is enabled by turning on the T_WOSO flag in the thread
mode.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>