add comments in the tests explaining what the ignore flags do

tests/run-pass/aux_test.rs

@@ -1,4 +1,5 @@
 // aux-build:dep.rs
+// This ignores the test against rustc, but runs it against miri:
 // ignore-cross-compile
 
 extern crate dep;

tests/run-pass/u128.rs

@@ -8,11 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-stage0
+// This disables the test completely:
 // ignore-stage1
 
-// ignore-emscripten
-
 #![feature(i128_type)]
 
 fn b<T>(t: T) -> T { t }