MINOR: debug: place the post_mortem struct in its own section.

Placing it in its own section will ease its finding, particularly in
gdb which is too dumb to find anything in memory. Now it will be
sufficient to issue this:

  $ gdb -ex "info files" -ex "quit" ./haproxy core 2>/dev/null |grep _post_mortem
  0x0000000000cfd300 - 0x0000000000cfe780 is _post_mortem

or this:

   $ objdump -h haproxy|grep post
    34 _post_mortem  00001480  0000000000cfd300  0000000000cfd300  008fc300  2**8

to spot the symbol's address. Then it can be read this way:

   (gdb) p *(struct post_mortem *)0x0000000000cfd300

src/debug.c

@@ -156,7 +156,7 @@ struct post_mortem {
 	 */
 	uint nb_components;              // # of components below
 	struct post_mortem_component *components; // NULL or array
-} post_mortem ALIGNED(256) = { };
+} post_mortem ALIGNED(256) HA_SECTION("_post_mortem") = { };
 
 unsigned int debug_commands_issued = 0;