libsysprof-capture: add frame type for tracing

This is like sample but has an "enter/exit" flag with it. This can be
useful when you want to provide tracing instead of sampling. We use a
different frame type so that we can denote that this isn't traditional
sampling, and the flag can be used to find the next exit for the current
enter for calculating durations.

The entire stack trace is provided to make things easier on tools
which may want to deal with indirect functions that were not instrumented
but can be unwound. That may allow for tooling to give the user some
insight that it's not *just* this function entering, but some functions
before it were entered too.

This also adds a SysprofTracer instrument which will preload a
libsysprof-tracer-6.so into the process providing the
__cyg_profile_func_enter() and __cyg_profile_func_leave() hooks.

src/libsysprof-capture/sysprof-collector.c

@@ -576,6 +576,43 @@ sysprof_collector_sample (SysprofBacktraceFunc  backtrace_func,
   } COLLECTOR_END;
 }
 
+void
+sysprof_collector_trace (SysprofBacktraceFunc  backtrace_func,
+                         void                 *backtrace_data,
+                         bool                  entering)
+{
+  COLLECTOR_BEGIN {
+    SysprofCaptureTrace *ev;
+    size_t len;
+
+    len = sizeof *ev + (sizeof (SysprofCaptureTrace) * MAX_UNWIND_DEPTH);
+
+    if ((ev = mapped_ring_buffer_allocate (collector->buffer, len)))
+      {
+        int n_addrs;
+
+        /* See comment from sysprof_collector_allocate(). */
+        if (backtrace_func)
+          n_addrs = backtrace_func (ev->addrs, MAX_UNWIND_DEPTH, backtrace_data);
+        else
+          n_addrs = 0;
+
+        ev->n_addrs = ((n_addrs < 0) ? 0 : (n_addrs > MAX_UNWIND_DEPTH) ? MAX_UNWIND_DEPTH : n_addrs);
+        ev->frame.len = sizeof *ev + sizeof (SysprofCaptureAddress) * ev->n_addrs;
+        ev->frame.type = SYSPROF_CAPTURE_FRAME_TRACE;
+        ev->frame.cpu = _do_getcpu ();
+        ev->frame.pid = collector->pid;
+        ev->frame.time = SYSPROF_CAPTURE_CURRENT_TIME;
+        ev->tid = collector->tid;
+        ev->entering = !!entering;
+        ev->padding1 = 0;
+
+        mapped_ring_buffer_advance (collector->buffer, ev->frame.len);
+      }
+
+  } COLLECTOR_END;
+}
+
 void
 sysprof_collector_mark (int64_t     time,
                         int64_t     duration,