Improve rtree cache with a two-level cache design.

Two levels of rcache is implemented: a direct mapped cache as L1, combined with
a LRU cache as L2.  The L1 cache offers low cost on cache hit, but could suffer
collision under circumstances.  This is complemented by the L2 LRU cache, which
is slower on cache access (overhead from linear search + reordering), but solves
collison of L1 rather well.

src/prof.c

@@ -310,6 +310,7 @@ prof_leave(tsd_t *tsd, prof_tdata_t *tdata) {
 }
 
 #ifdef JEMALLOC_PROF_LIBUNWIND
+JEMALLOC_ALIGNED(CACHELINE)
 void
 prof_backtrace(prof_bt_t *bt) {
 	int nframes;