Add JEMALLOC_COLD, and mark some functions cold.

This hints to the compiler that it should care more about space than CPU (among
other things).  In cases where the compiler lacks profile-guided information,
this can be a substantial space savings.

For now, we mark the mallctl or atexit driven profiling and stats functions that
take up the most space.

src/prof_recent.c

@@ -528,6 +528,7 @@ prof_recent_alloc_dump_node(emitter_t *emitter, prof_recent_t *node) {
 }
 
 #define PROF_RECENT_PRINT_BUFSIZE 65536
+JEMALLOC_COLD
 void
 prof_recent_alloc_dump(tsd_t *tsd, write_cb_t *write_cb, void *cbopaque) {
 	cassert(config_prof);