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/malloc_io.c

@@ -321,6 +321,7 @@ x2s(uintmax_t x, bool alt_form, bool uppercase, char *s, size_t *slen_p) {
 	return s;
 }
 
+JEMALLOC_COLD
 size_t
 malloc_vsnprintf(char *str, size_t size, const char *format, va_list ap) {
 	size_t i;