profiling: Implement a simple task exit notifier when disabled

profiling: Implement a simple task exit notifier when disabled

Some kernel code currently depends on the profiling subsystem solely for
its task exit notifier. The rest of the profiling subsystem is left
unused.

Add a simple task exit notifier implemented in kernel/exit.c to make
such code work properly even when profiling is disabled. This will allow
unnecessary profiling code to be disabled without breakage.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>