Add MALLCTL_ARENAS_ALL.

Add the MALLCTL_ARENAS_ALL cpp macro as a fixed index for use
in accessing the arena.<i>.{purge,decay,dss} and stats.arenas.<i>.*
mallctls, and deprecate access via the arenas.narenas index (to be
removed in 6.0.0).

include/jemalloc/internal/util.h

@@ -54,6 +54,10 @@
  */
 #define	JEMALLOC_ARG_CONCAT(...) __VA_ARGS__
 
+/* cpp macro definition stringification. */
+#define	STRINGIFY_HELPER(x) #x
+#define	STRINGIFY(x) STRINGIFY_HELPER(x)
+
 /*
  * Silence compiler warnings due to uninitialized values.  This is used
  * wherever the compiler fails to recognize that the variable is never used