Add tracepoint_disable_destructors()

Calling this function from an instrumented program allows disabling
tracepoint destructors. This allows threads to continue calling
tracepoint code even after the tracepoint destructors have run. This is
needed for applications that exit without joining all their threads.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Jeffrey Chen <cpthk@live.com>