Add thapi_start() and thapi_stop()

[thilinarmtb]

I created lttng_ust_toggle event category and implemented thapi_start() and thapi_stop()
(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_stop() 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)).

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