Add thapi_start() and thapi_stop()

[thilinarmtb]

I created lttng_ust_toggle event category and implemented thapi_start() and thapi_stop()
user APIs (in a similar spirit to cudaProfilerStart() etc.). The library which the user has to link
against is named libThapi.so.

Usage of the API is as follows:

#include <thapi.h>
//
// code
//
thapi_start();
// code to be profiled
thapi_stop();
//
// more code
//

If libThapi.so is linked, thapi_auto_stop() (which is not part of the user API) is called automatically
during the library load time using __attribute__((constructor)). I added a check in configure.ac to
check if the compiler supports __attribute__((constructor)).

If the user runs iprof with --toggle-on, the traces generated by thapi_auto_stop(), thapi_start()
and thapi_stop() will be respected by babeltrace (otherwise there is no change to iprof/babeltrace
behavior). Since thapi_auto_stop() is called automatically, babeltrace will filter out all of the traces
till it see a thapi_start(). Trace filtering out will start again once the user call thapi_stop(). This API
only affects the trace collection of the calling process/thread.

P.S: This feature requires lttng-ust >= 2.12.8. Specifically, this fix.

TODO

• Respect lttng_ust_profiler_* lttng_ust_toggle_* events when calculating statistics
• Fix failing checks
• Add the filter.toggle.toggle to babeltrace graph

[thilinarmtb]

@TApplencourt : Two tests fail out of the three tests used to check the toggle API. Those two fail
because we have not integrated the toggle plugin to babeltrace_thapi. Once we do that, I think
those two will pass as well.

[TApplencourt]

I can show you how to do it tomorrow : )

[thilinarmtb]

I can show you how to do it tomorrow : )

Works for me.