Add JEMALLOC_ALLOC_JUNK and JEMALLOC_FREE_JUNK macros

Replace hardcoded 0xa5 and 0x5a junk values with JEMALLOC_ALLOC_JUNK and
JEMALLOC_FREE_JUNK macros, respectively.

include/jemalloc/internal/util.h

@@ -40,6 +40,10 @@
  */
 #define	MALLOC_PRINTF_BUFSIZE	4096
 
+/* Junk fill patterns. */
+#define	JEMALLOC_ALLOC_JUNK	0xa5
+#define	JEMALLOC_FREE_JUNK	0x5a
+
 /*
  * Wrap a cpp argument that contains commas such that it isn't broken up into
  * multiple arguments.