Ignore a spuriously failing test on asmjs

Other tests are already ignored for missing `rust_begin_unwind`, let's add
another.

src/test/run-pass/vec-macro-no-std.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-emscripten missing rust_begin_unwind
+
 #![feature(lang_items, start, libc, alloc)]
 #![no_std]