alloc_jemalloc: don’t assume MIN_ALIGN for small sizes

See previous commit’s message for what is expected of allocators
in general, and https://github.com/jemalloc/jemalloc/issues/1072
for discussion of what jemalloc does specifically.

src/liballoc/tests/heap.rs

@@ -20,6 +20,11 @@ fn alloc_system_overaligned_request() {
     check_overalign_requests(System)
 }
 
+#[test]
+fn std_heap_overaligned_request() {
+    check_overalign_requests(Heap)
+}
+
 fn check_overalign_requests<T: Alloc>(mut allocator: T) {
     let size = 8;
     let align = 16; // greater than size