Skip a memory-hungry test that OOMs

Attempting to fix https://travis-ci.org/rust-lang/rust/jobs/377407894 via some
selective ignoring tests

src/liballoc/tests/str.rs

@@ -479,6 +479,7 @@ mod slice_index {
     }
 
     #[test]
+    #[cfg(not(target_arch = "asmjs"))] // hits an OOM
     fn simple_big() {
         fn a_million_letter_x() -> String {
             let mut i = 0;