posix: Use interal mutex for once implementation

Enable pthread_once() for all configurations.  The pthread_once()
function is one means to initialize POSIX keys.  Another use case is the
C++ support.
score: Delete _Assert_Thread_dispatching_repressed

Add _Debug_Is_thread_dispatching_allowed().  This makes it possible to
assert the opposite.

Use _ISR_Disable_without_giant()/_ISR_Enable_without_giant() to avoid
misleading secondary assertion failures.
score: Delete _Assert_Owner_of_giant()

Add _Debug_Is_owner_of_giant().  This makes it possible to assert the
opposite.
score: Add _Debug_Is_owner_of_allocator()
score: Add _Assert_Not_reached()
psxtests: move pthread_once tests into an extra test.
psxonce01: Add call counter to check if init function has been called.