Fix imports in ctap, embedded_flash and lang-items.

third_party/lang-items/src/lib.rs

@@ -7,3 +7,10 @@ mod allocator;
 mod panic_handler;
 #[cfg(not(feature = "std"))]
 mod util;
+
+#[cfg(feature = "std")]
+#[no_mangle]
+unsafe fn libtock_alloc_init(_app_heap_start: usize, _app_heap_size: usize) {
+    // Stub so that the symbol is present.
+    unimplemented!()
+}