FIN: ignore failing test on emscripten, see rust-lang#41299

Author: TimNN

src/test/run-pass/issue-29663.rs

@@ -10,6 +10,8 @@
 
 // write_volatile causes an LLVM assert with composite types
 
+// ignore-emscripten See #41299: probably a bad optimization
+
 #![feature(volatile)]
 use std::ptr::{read_volatile, write_volatile};