Allow setting custom backtrace hook

Existing backtrace implementations skip native stack frames from runtimes like
Python. The hook allows to augment the backtraces to attribute allocations to
native functions in heap profiles.

test/unit/prof_hook.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+if [ "x${enable_prof}" = "x1" ] ; then
+  export MALLOC_CONF="prof:true,lg_prof_sample:0"
+fi
+