DTrace support: function calls, GC activity, line execution

Tested on macOS 10.11 dtrace, Ubuntu 16.04 SystemTap, and libbcc.

Largely based by an initial patch by Jesús Cea Avión, with some
influence from Dave Malcolm's SystemTap patch and Nikhil Benesch's
unification patch.

Things deliberately left out for simplicity:
- ustack helpers, I have no way of testing them at this point since
they are Solaris-specific
- PyFrameObject * in function__entry/function__return, this is
SystemTap-specific
- SPARC support
- dynamic tracing
- sys module dtrace facility introspection

All of those might be added later.

Modules/gcmodule.c

@@ -25,6 +25,7 @@
 
 #include "Python.h"
 #include "frameobject.h"        /* for PyFrame_ClearFreeList */
+#include "pydtrace.h"
 #include "pytime.h"             /* for _PyTime_GetMonotonicClock() */
 
 /* Get an object's GC head */
@@ -925,6 +926,9 @@ collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
         PySys_WriteStderr("\n");
     }
 
+    if (PyDTrace_GC_START_ENABLED())
+        PyDTrace_GC_START(generation);
+
     /* update collection and allocation counters */
     if (generation+1 < NUM_GENERATIONS)
         generations[generation+1].count += 1;
@@ -1069,6 +1073,10 @@ collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
     stats->collections++;
     stats->collected += m;
     stats->uncollectable += n;
+
+    if (PyDTrace_GC_DONE_ENABLED())
+        PyDTrace_GC_DONE(n+m);
+
     return n+m;
 }