Add size class computation capability.

Add size class computation capability, currently used only as validation
of the size class lookup tables.  Generalize the size class spacing used
for bins, for eventual use throughout the full range of allocation
sizes.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -47,6 +47,11 @@
  */
 #undef JE_FORCE_SYNC_COMPARE_AND_SWAP_8
 
+/*
+ * Defined if __builtin_clz() and __builtin_clzl() are available.
+ */
+#undef JEMALLOC_HAVE_BUILTIN_CLZ
+
 /*
  * Defined if OSSpin*() functions are available, as provided by Darwin, and
  * documented in the spinlock(3) manual page.