DEBUG: mark ABORT_NOW() as unreachable

The purpose is to make the program die at this point, so let's help the
compiler optimise the code (especially in sensitive areas) by telling it
that ABORT_NOW() does not return. This reduces the overall code size by
~0.5%.
diff --git a/include/haproxy/bug.h b/include/haproxy/bug.h
index ddf55e8..4cedbf1 100644
--- a/include/haproxy/bug.h
+++ b/include/haproxy/bug.h
@@ -47,7 +47,7 @@
 /* More efficient than abort() because it does not mangle the
   * stack and stops at the exact location we need.
   */
-#define ABORT_NOW() do { DUMP_TRACE(); (*(volatile int*)1=0); } while (0)
+#define ABORT_NOW() do { DUMP_TRACE(); (*(volatile int*)1=0); my_unreachable(); } while (0)
 #endif
 
 /* This is the generic low-level macro dealing with conditional warnings and