Darwin malloc_size override support proposal.

Darwin has similar api than Linux/FreeBSD's malloc_usable_size.

test/unit/junk.c

@@ -30,7 +30,7 @@ do_allocs(size_t size, bool zero, size_t lg_align) {
 		if (opt_junk_alloc && !zero) {				\
 			expect_ptr_eq(ptr, last_junked_ptr, "");	\
 			expect_zu_eq(last_junked_usize,			\
-			    malloc_usable_size(ptr), "");		\
+			    TEST_MALLOC_SIZE(ptr), "");			\
 		}							\
 	} while (0)
 	if (!zero && lg_align == 0) {

test/unit/prof_stats.c

@@ -43,7 +43,7 @@ test_combinations(szind_t ind, size_t sizes_array[N_PTRS],
 		int flags = flags_array[i];
 		void *p = mallocx(sz, flags);
 		assert_ptr_not_null(p, "malloc() failed");
-		assert(malloc_usable_size(p) == sz_index2size(ind));
+		assert(TEST_MALLOC_SIZE(p) == sz_index2size(ind));
 		ptrs[i] = p;
 		live_req_sum += sz;
 		live_count++;