Fast back-trace for x86_64 for only collecting the call stack.

Adds new function to perform a pure stack walk without unwinding,
functionally similar to backtrace() but accelerated by an address
attribute cache the caller maintains across calls.
diff --git a/src/dwarf/Gparser.c b/src/dwarf/Gparser.c
index 13bd9a2..1db1546 100644
--- a/src/dwarf/Gparser.c
+++ b/src/dwarf/Gparser.c
@@ -803,6 +803,10 @@
 	       __FUNCTION__, (long) c->ip);
       return -UNW_EBADFRAME;
     }
+
+  if (c->stash_frames)
+    tdep_stash_frame (c, rs);
+
   return 0;
 }